title: "JavaScript String, Array and Object" description: "" added: "Aug 6 2020" tags: [js]
Note that JavaScript distinguishes between String objects and primitive string values (The same is true of Boolean and Numbers). String literals and strings returned from String calls in a non-constructor context (called without using the new keyword) are primitive strings. In contexts where a method is to be invoked on a primitive string or a property lookup occurs, JavaScript will automatically wrap the string primitive and call the method or perform the property lookup on the wrapper object instead.
A String object can always be converted to its primitive counterpart with the valueOf() method.
const strPrim = "foo";
const strPrim2 = String(1);
const strObj = new String(strPrim); // You should rarely use String as a constructor
console.log(typeof strPrim); // "string"
console.log(typeof strPrim2); // "string"
console.log(typeof strObj); // "object"
console.log(strObj.valueOf());
`string text ${expression} string text`
// Multi-line string
// newline characters inserted in the source are part of the template literal
console.log(`string text line 1
string text line 2`);
// "string text line 1
// string text line 2"
A more advanced form of template literals are tagged templates. Tags allow you to parse template literals with a function, which you can manipulate before outputting. The first argument is a string array containing string literals from the template: First element in the array is string starting from index 0 to the first interpolated value, second element is string after first interpolated value to next interpolation and so on until end of template is reached. All the interpolated expressions are evaluated and passed to the tag as second argument in order of their occurrence. In the end, your function can return your manipulated string or it can return something completely different.
The special raw property, available on the first argument to the tag function, allows you to access the raw strings as they were entered, without processing escape sequences.
var person = 'Mike';
var age = 28;
function myTag(strings, ...keys) {
var str0 = strings[0]; // "That "
var str1 = strings[1]; // " is a "
var str2 = strings[2]; // There is an empty string after the final expression
var ageStr = keys[1] > 99 ? 'centenarian' : 'youngster';
return `${str0}${keys[0]}${str1}${ageStr}`;
}
var output = myTag`That ${ person } is a ${ age }`;
console.log(output); // That Mike is a youngster
// raw property
function tag(strings) {
console.log(strings[0]); // escape \n to a new line
console.log(strings.raw[0]); // include \n in the string
}
tag`string text line 1 \n string text line 2`;
str.startsWith(searchString[, position])
str.endsWith(searchString[, length])
str.padStart(targetLength [, padString])
str.padEnd(targetLength [, padString])
str.repeat(count)
const str = 'To be, or not to be, that is the question.'
str.startsWith('To be') // true
str.startsWith('not to be', 10) // true
str.endsWith('question.') // true
str.endsWith('to be', 19) // true
'abc'.padStart(10, "foo") // "foofoofabc"
'abc'.padStart(6, "123465") // "123abc"
'abc'.padStart(1) // "abc"
'abc'.padEnd(10) // "abc "
'abc'.padEnd(10, "foo") // "abcfoofoof"
'abc'.padEnd(6, "123456") // "abc123"
'abc'.repeat(-1) // RangeError
'abc'.repeat(0) // ''
'abc'.repeat(1) // 'abc'
'abc'.repeat(2) // 'abcabc'
It returns a new string with some or all matches of a pattern replaced by a replacement. The pattern can be a string or a RegExp, and the replacement can be a string or a function. The function's result (return value) will be used as the replacement string. The original string is left unchanged. If not using replaceAll and the pattern is a string, only the first occurrence will be replaced.
The replacement string can include the following special replacement patterns:
$&: the matched substring$\: the portion of the string that precedes the matched substring$': the portion of the string that follows the matched substring$n: the nth parenthesized submatch string'John Smith'.replace(/(\w+)\s(\w+)/, '$2, $1'); // Smith, John
'abc5885c'.replace(/(\d)(\d)\2\1/g, '-$&-'); // abc-5885-c
// When using a regular expression search value, must set the global flag
'aabbcc'.replaceAll(/b/g, '.'); // aa..cc
// the matched substring
// capture groups
// offset of the matched substring within the whole string being examined
// the whole string being examined
function replacer(match, p1, p2, p3, offset, string) {
// p1 is nondigits, p2 digits, and p3 non-alphanumerics
return [p1, p2, p3].join('-');
}
'abc123#$'.replace(/([^\d]*)(\d*)([^\w]*)/, replacer); // abc-123-#$
Javascript makes it easy to hide the real cost of string comparisons. If you need to compare strings in C, you’d use the strcmp(a, b) function. Javascript uses === instead, so you don’t see the strcmp. But it’s there, and a string comparison will usually require comparing each of the characters in the string with the ones in the other string; string comparison is O(n). With the advent of TypeScript this should be easily avoidable, as enums are integers by default.
Integers are usually passed by value in JS engines, whereas strings are always passed as pointers, and memory accesses are expensive.
// No
enum Position {
TOP = 'TOP',
BOTTOM = 'BOTTOM',
}
// Yes
enum Position {
TOP, // = 0
BOTTOM, // = 1
}
Note that
value instanceof Arrayevaluates tofalsewhen value is an array created in a different iframe than the Array constructor function (it is an instance ofthatFrame.contentWindow.Array).
({}).toString.call([]); // '[object Array]'
({}).toString.call({}); // '[object Object]'
({}).toString.call(''); // '[object String]'
({}).toString.call(null); // '[object Null]'
// liner time, but has at least two drawbacks:
// 1. doesn't distinguish numbers and "numeric strings" like [1, "1"]
// 2. all objects will be considered equal `[object Object]`
function uniq(a) {
var seen = {};
return a.filter(function(item) {
return seen.hasOwnProperty(item) ? false : (seen[item] = true);
});
}
Note that JSON.parse(JSON.stringify(arr)) depends upon JSON, it also inherits its limitations. undefined, Function, and Symbol are not valid JSON values. If any such values are encountered during the stringify conversion, they are either omitted (when found in an object) or changed to null (when found in an array). Btw, Dates will be turned into strings, Sets and Maps would be converted to empty objects.
JSON.stringify(function(){}) // undefined
JSON.stringify([undefined, function(){}, () => {}]) // "[null, null, null]"
const obj = {
set: new Set([1,2]),
map: new Map([
["a", 1],
["b", 2],
])
}
JSON.stringify(obj) // '{"set":{},"map":{}}'
In practice, such object is expected to actually have a length property and to have indexed elements in the range 0 to length - 1. You can actually just use Array's slice function to convert it into a standard JavaScript array. The slice function is intentionally generic; it does not require that its this value be an Array object, so it works on anything that has a length property. Many DOM objects are array-like — for example, NodeList and HTMLCollection. The arguments object is also array-like.
Array.prototype.slice.call(obj); // same as [].slice.call(obj)
It creates a new, shallow-copied Array instance from an array-like or iterable object. It has an optional parameter mapFn, which allows you to execute a map function on each element of the array that is being created.
Array.from('foo'); // [ "f", "o", "o" ]
const set = new Set(['foo', 'bar', 'baz', 'foo']);
Array.from(set); // [ "foo", "bar", "baz" ]
Array.from([1, 2, 3], x => x + x); // [2, 4, 6]
// Since the array is initialized with `undefined` on each position,
// the value of `v` below will be `undefined`
Array.from({ length: 5 }, (v, i) => i); // [0, 1, 2, 3, 4]
It creates a new Array instance from a variable number of arguments. The difference between Array.of() and the Array constructor is in the handling of integer arguments: Array.of(7) creates an array with a single element, 7, whereas Array(7) creates an empty array with a length of 7 (which implies an array of 7 empty slots, not slots with actual undefined values).
Array.of(7); // [7]
Array.of(1, 2, 3); // [1, 2, 3]
Array(7); // array of 7 empty slots
Array(1, 2, 3); // [1, 2, 3]
Array()can be called with or withoutnew. Both create a new Array instance.- The
lengthproperty of an array is a 32-bit unsigned integer, which limits the maximum number of entries an array can have, which isMath.pow(2, 32) - 1.- Arrays can contain "empty slots" (not the same as slots filled with the value
undefined), which are called sparse arrays.
The fill method takes up to three arguments value, start and end. The start and end arguments are optional with default values of 0 and the length of the this object. fill() is intentionally generic, it does not require that its this value be an Array object.
Array(3).fill(4) // [4, 4, 4]
// Objects by reference
var arr = Array(3).fill({}) // [{}, {}, {}];
arr[0].hi = "hi"; // [{ hi: "hi" }, { hi: "hi" }, { hi: "hi" }]
[].fill.call({ length: 3 }, 4); // {0: 4, 1: 4, 2: 4, length: 3}
// compare with Array.from
const array = Array.from({ length: 3 }, () => ({}));
array[0].hi = "hi"; // [{ hi: "hi" }, {}, {}]
arr.find(callback(element[, index[, array]])[, thisArg])
arr.findIndex(callback(element[, index[, array]])[, thisArg])
const array1 = [5, 12, 8, 130, 44];
const found = array1.find(element => element > 10); // 12
const foundIndex = array1.findIndex(element => element > 10); // 1
const inventory = [
{name: 'apples', quantity: 2},
{name: 'bananas', quantity: 0},
{name: 'cherries', quantity: 5}
];
const result = inventory.find( ({ name }) => name === 'cherries' );
initialValue is provided, then accumulator will be equal to initialValue, and currentValue will be equal to the first value in the array. If no initialValue is provided, then accumulator will be equal to the first value in the array, and currentValue will be equal to the second. arr.reduce(callback(accumulator, currentValue[, index[, array]])[, initialValue])
// implement map using reduce
function implementMapUsingReduce(list, func) {
return list.reduce((acc, cur, i) => {
acc[i] = func(cur);
return acc;
}, []);
}
It changes the contents of an array by removing or replacing existing elements or adding new elements in place.
var arrDeletedItems = array.splice(start[, deleteCount[, item1[, item2[, ...]]]])
// insert
var myFish = ['angel', 'clown', 'mandarin', 'sturgeon'];
var removed = myFish.splice(2, 0, 'drum');
// remove
var myFish = ['angel', 'clown', 'drum', 'mandarin', 'sturgeon'];
var removed = myFish.splice(3, 1);
// replace
var myFish = ['angel', 'clown', 'trumpet', 'sturgeon'];
var removed = myFish.splice(0, 2, 'parrot', 'anemone', 'blue');
Move an array item to a different position.
export function arrayMoveMutable(array, fromIndex, toIndex) {
const startIndex = fromIndex < 0 ? array.length + fromIndex : fromIndex;
if (startIndex >= 0 && startIndex < array.length) {
const endIndex = toIndex < 0 ? array.length + toIndex : toIndex;
const [item] = array.splice(fromIndex, 1);
array.splice(endIndex, 0, item);
}
}
export function arrayMoveImmutable(array, fromIndex, toIndex) {
const newArray = [...array];
arrayMoveMutable(newArray, fromIndex, toIndex);
return newArray;
}
const arr1 = [1, 2, [3, 4]];
arr1.flat(); // [1, 2, 3, 4]
const arr2 = [1, 2, [3, 4, [5, 6]]];
arr2.flat(); // [1, 2, 3, 4, [5, 6]]
const arr3 = [1, 2, [3, 4, [5, 6, [7, 8, [9, 10]]]]];
arr3.flat(Infinity); // [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
// deep level flatten use recursion
// [].concat([1,[2,3]]) returns [1,[2,3]]
// [].concat(1,[2,3]) returns [1,2,3]
function flattenDeep(arr) {
return arr.reduce((acc, val) => Array.isArray(val) ? acc.concat(flattenDeep(val)) : acc.concat(val), []);
}
// flatMap is identical to a map() followed by a flat() of depth 1
let arr1 = ["it's Sunny in", "", "California"];
// [["it's","Sunny","in"],[""],["California"]]
arr1.map(x => x.split(" "));
// ["it's","Sunny","in", "", "California"]
arr1.flatMap(x => x.split(" "));
sort() method sorts the elements of an array in place and returns the reference to the same array.Array.prototype.sort is stable. All major JavaScript engines now implement a stable Array sort.Array.prototype.at() takes an integer value and returns the item at that index. Negative integers count back from the last item in the array.
Array.prototype.with(index, value) changes the value of a given index in the array, returning a new array with the element at the given index replaced with the given value. The empty slots in the source array will be replaced with undefined in the new array.
Array.prototype.toReversed() returns a new array with the elements in reversed order.
Array.prototype.toSorted() returns a new array with the elements sorted in ascending order.
Array.prototype.toSpliced(start, deleteCount, ...) returns a new array with some elements removed or replaced at a given index, rather than modifying the original array.
function badEmptyCheck(value) {
return Object.keys(value).length === 0;
}
badEmptyCheck(new Object()); // true
badEmptyCheck(new String()); // true
badEmptyCheck(new Array()); // true
function goodEmptyCheck(value) {
return Object.keys(value).length === 0 && value.constructor === Object;
}
badEmptyCheck(new Object()); // true
badEmptyCheck(new String()); // false
badEmptyCheck(new Array()); // false
It defines a new property or modifies an existing property on an object, and returns the object. By default, values added using Object.defineProperty() are immutable and not enumerable. Property descriptors present in objects come in two main flavors: data descriptors or accessor descriptors. A data descriptor is a property that has a value, which may or may not be writable. An accessor descriptor is a property described by a getter-setter pair of functions. A descriptor must be one of these two flavors; it cannot be both.
// data descriptor
Object.defineProperty(o, 'key', {
enumerable: false,
configurable: false,
writable: true,
value: 'static'
});
// accessor descriptor
var bValue = 38;
Object.defineProperty(o, 'b', {
enumerable: true,
configurable: true,
get() { return bValue; },
set(newValue) { bValue = newValue; }
});
o.b; // 38, the value of o.b is now always identical to bValue
var o = {};
o.a = 1;
// is equivalent to:
Object.defineProperty(o, 'a', {
value: 1,
writable: true,
configurable: true,
enumerable: true
});
// On the other hand,
// default: not enumerable, not configurable, not writable
Object.defineProperty(o, 'a', { value: 1 });
// is equivalent to:
Object.defineProperty(o, 'a', {
value: 1,
writable: false,
configurable: false,
enumerable: false
});
var o = {};
Object.defineProperty(o, 'a', {
value: 37,
writable: false
});
o.a = 25; // it would throw error in strict mode even if the value had been the same
console.log(o.a); // 37
var o = {};
Object.defineProperty(o, 'a', {
value: 1,
enumerable: true
});
Object.defineProperty(o, 'b', {
value: 2,
enumerable: false
});
Object.defineProperty(o, 'c', {
value: 3
});
o.d = 4;
// non-enumerable means that property will not be shown in Object.keys() or for-loop
for (var i in o) { console.log(i) } // logs 'a' and 'd'
Object.keys(o); // ['a', 'd']
o.propertyIsEnumerable('a'); // true
o.propertyIsEnumerable('b'); // false
var p = { ...o };
p.a // 1
p.b // undefined
The get syntax binds an object property to a function that will be called when that property is looked up. The set syntax binds an object property to a function to be called when there is an attempt to set that property.
// getter
const obj = {
log: ['example', 'test'],
get latest() {
return this.log[this.log.length - 1];
}
}
console.log(obj.latest); // "test"
// setter
const language = {
set current(name) {
this.log.push(name);
},
log: []
}
language.current = 'EN';
console.log(language.log); // ['EN']
// a general purpose memoizer
function lazy<T>(getter: () => T): { value: T } {
return {
get value() {
const value = getter();
Object.defineProperty(this, 'value', { value });
return value;
},
};
}
const myValue = lazy(() => 'Hello, world');
myValue.value; // compute value
myValue.value; // return cached value
for...in loop (the difference being that for-in loop enumerates properties in the prototype chain as well).[key, value] pairs found directly upon object.A for...in loop only iterates over enumerable, non-Symbol properties. Objects created from built–in constructors like Object and String have inherited non–enumerable properties from Object.prototype and String.prototype, such as String's indexOf() method or Object's toString() method (not enumerable). The loop will iterate over all enumerable properties of the object itself and those enumerable properties the object inherits from its prototype chain.
var arr = ['a', 'b', 'c'];
console.log(Object.keys(arr)); // ['0', '1', '2']
var obj = { foo: 'bar', baz: 42 };
console.log(Object.values(obj)); // ['bar', 42]
const obj = { foo: 'bar', baz: 42 };
console.log(Object.entries(obj)); // [ ['foo', 'bar'], ['baz', 42] ]
// fromEntries() method transforms a list of key-value pairs into an object
// iterable argument is expected
const arr = [ ['0', 'a'], ['1', 'b'], ['2', 'c'] ];
Object.fromEntries(arr); // { 0: "a", 1: "b", 2: "c" }
const map = new Map([ ['foo', 'bar'], ['baz', 42] ]);
Object.fromEntries(map); // { foo: "bar", baz: 42 }
// hasOwnProperty() returns a boolean indicating whether the object has the specified property
// as its own property as opposed to inheriting it
const o = new Object();
o.prop = 'exists';
o.hasOwnProperty('prop'); // true
o.hasOwnProperty('toString'); // false
// getOwnPropertyNames() returns an array of all properties (including non-enumerable properties)
// found directly in a given object
const arr = ['a', 'b', 'c'];
console.log(Object.getOwnPropertyNames(arr).sort()); // ["0", "1", "2", "length"]
// getPrototypeOf() returns the prototype (the value of the internal [[Prototype]] property)
// of the specified object
var proto = {};
var obj = Object.create(proto);
Object.getPrototypeOf(obj) === proto; // true
// isPrototypeOf() checks if an object exists within another object's prototype chain
function Foo() {}
function Bar() {}
function Baz() {}
Bar.prototype = Object.create(Foo.prototype);
Baz.prototype = Object.create(Bar.prototype);
var baz = new Baz();
Baz.prototype.isPrototypeOf(baz); // true
Bar.prototype.isPrototypeOf(baz); // true
Foo.prototype.isPrototypeOf(baz); // true
Object.prototype.isPrototypeOf(baz); // true
Every object has a toString() method that is automatically called when the object is to be represented as a text value or when an object is referred to in a manner in which a string is expected. For Numbers, toString() takes an optional parameter radix, the value of radix must be minimum 2 and maximum 36.
You can create a function to be called in place of the default toString() method. The toString() method you create can be any value you want, but it will be most useful if it carries information about the object.
function Dog(name, breed, color, sex) {
this.name = name;
this.breed = breed;
this.color = color;
this.sex = sex;
}
theDog = new Dog('Gabby', 'Lab', 'chocolate', 'female');
theDog.toString(); // [object Object]
Dog.prototype.toString = function() {
return `Dog ${this.name} is a ${this.sex} ${this.color} ${this.breed}`;
}
theDog.toString(); // "Dog Gabby is a female chocolate Lab"
JavaScript calls valueOf() to convert an object to a primitive value. You rarely need to invoke the valueOf method yourself; JavaScript automatically invokes it when encountering an object where a primitive value is expected. A unary plus sign can sometimes be used as a shorthand for valueOf.
+"5" // 5
+"" // 0
+"foo" // NaN
+{} // NaN
+[] // 0
+[1] // 1
+[1,2] // NaN
+undefined // NaN
+null // 0
+true // 1
+false // 0
Copy all enumerable own properties from one or more source objects to a target object and return the modified target object. If the source value is a reference to an object, it only copies that reference value (shallow copy). Properties in the target object will be overwritten by properties in the sources if they have the same key.
var obj = { a: 1 };
var copy = Object.assign({}, obj);
console.log(copy); // { a: 1 }
// Merging objects
var o1 = { a: 1 };
var o2 = { b: 2 };
var o3 = { c: 3 };
var obj = Object.assign(o1, o2, o3);
console.log(obj); // { a: 1, b: 2, c: 3 }
console.log(o1); // { a: 1, b: 2, c: 3 }
obj === o1; // true
It creates a new object, using an existing object as the prototype of the newly created object.
const o1 = Object.create({}); // create a normal object
const o2 = Object.create(null); // create a totally empty object (without prototype)
// Is equivalent to:
o2 = { __proto__: null };
"first is: " + o1 // "first is: [object Object]"
"second is: " + o2 // throws error: Cannot convert object to primitive value
o1.toString() // [object Object]
o2.toString() // throws error: ocn.toString is not a function
o1.constructor // "Object() { [native code] }"
o2.constructor // "undefined"
// Class inheritance with Object.create()
function Shape() {
this.x = 0;
this.y = 0;
}
function Rectangle() {
// call super constructor
Shape.call(this);
}
Rectangle.prototype = Object.create(Shape.prototype);
// If don't set constructor to Rectangle, it will take Shape as the constructor
Rectangle.prototype.constructor = Rectangle;
var rect = new Rectangle();
rect instanceof Rectangle // true
rect instanceof Shape // true
// ** In modern code, the class syntax should be preferred in any case.**
Freezing an object prevents extensions (An object is extensible if new properties can be added to it) and makes existing properties non-writable and non-configurable. A frozen object can no longer be changed: new properties cannot be added, existing properties cannot be removed, their enumerability, configurability, writability, or value cannot be changed, and the object's prototype cannot be re-assigned. freeze() returns the same object that was passed into the function. It does not create a frozen copy.
The result of calling Object.freeze(obj) only applies to the immediate properties of object itself (freeze is shallow).
const obj = {
prop() {},
foo: "bar",
};
const o = Object.freeze(obj);
o === obj; // true
obj.foo = "quux"; // silently does nothing
obj.quaxxor = "the friendly duck"; // silently doesn't add the property
// Attempted changes through Object.defineProperty, throw a TypeError in strict mode.
Object.defineProperty(obj, "foo", { value: "eit" });
// protect against prototype pollution
[
Object,
Object.prototype,
Function,
Function.prototype,
Array,
Array.prototype,
String,
String.prototype,
Number,
Number.prototype,
Boolean,
Boolean.prototype,
].forEach(Object.freeze);
It determines whether two values are the same value. The only difference between Object.is() and === is in their treatment of signed zeros and NaN values.
Object.is('foo', 'foo'); // true
var foo = { a: 1 };
var bar = { a: 1 };
Object.is(foo, bar); // false
+0 === -0 // true
Object.is(+0, -0); // false
NaN === NaN // false
Object.is(NaN, NaN); // true
Object.is(NaN, 0/0); // true
The Map object holds key-value pairs and remembers the original insertion order of the keys. Any value (both objects and primitive values) may be used as either a key or a value.
Object is similar to Map, and Objects have been used as Maps historically; however, there are important differences that make using a Map preferable in certain cases:
size property, while the number of properties in an Object must be determined manually. (A Map is iterable, whereas a objects is not iterable.)var myMap = new Map();
myMap.set(0, 'zero');
myMap.set(1, 'one');
myMap.size;
var myMap2 = new Map([
['key1', 'value1'],
['key2', 'value2']
]);
myMap.get('key1');
myMap.has('key1');
myMap.delete('key1');
// Iterating with for..of
for (let key of myMap.keys()) console.log(key);
for (let value of myMap.values()) console.log(value);
for (let [key, value] of myMap.entries()) console.log(key + ' = ' + value);
myMap.forEach(function(value, key, map) {
console.log(`map.get('${key}') = ${value}`);
});
myMap.clear();
// Relation with Array
// Use the Array.from to transform a map into a 2D key-value Array
Array.from(myMap);
// or
[...myMap];
// Or use the keys or values iterators and convert them to an array
Array.from(myMap.keys());
// Maps can be merged
var first = new Map([
[1, 'one'],
[2, 'two'],
[3, 'three'],
]);
var second = new Map([
[1, 'uno'],
[2, 'dos']
]);
// Merge two maps. The last repeated key wins.
var merged = new Map([...first, ...second]);
The Set object lets you store unique values of any type, whether primitive values or object references.
var mySet = new Set();
var mySet = new Set(['value1', 'value2', 'value3']);
// You can't add multiple elements to a set in one add()
mySet.add(1);
mySet.add(2);
mySet.has(1); // true
mySet.has(3); // false
mySet.delete(1);
mySet.size;
// There are no keys in Set, so key and value are the same here
for (let item of mySet.keys()) console.log(item);
for (let item of mySet.values()) console.log(item);
for (let [key, value] of mySet.entries()) console.log(key);
mySet.forEach(function(value, key, set) {
console.log(value);
});
mySet.clear();
// converting between Set and Array
mySet2 = new Set([1, 2, 3, 4]);
Array.from(mySet2)
// or
[...mySet2];
// intersection
var intersection = new Set([...set1].filter(x => set2.has(x)));
// Newly available 2024
const odds = new Set([1, 3, 5, 7, 9]);
const squares = new Set([1, 4, 9]);
console.log(odds.intersection(squares)); // Set(2) { 1, 9 }
console.log(odds.difference(squares)); // Set(3) { 3, 5, 7 }
console.log(odds.union(squares)); // Set(6) { 1, 3, 5, 7, 9, 4 }
An object's presence as a key in a WeakMap does not prevent the object from being garbage collected. Every key of a WeakMap is an object. Primitive data types as keys are not allowed. WeakMap allows garbage collector to do its task but not Map. There is no such thing as a list of WeakMap keys, they are just references to another objects.
In WeakMaps, references to key objects are held "weakly", which means that they do not prevent garbage collection when there would be no other reference to the object. Because of references being weak, you cannot iterate over its keys or values, cannot clear all items (no clear method), cannot check its size (no size property).
// Map and Weakmap
var k1 = {a: 1};
var k2 = {b: 2};
var map = new Map();
var wm = new WeakMap();
map.set(k1, 'k1');
wm.set(k2, 'k2');
k1 = null;
map.forEach(function (val, key) {
console.log(key, val); // {a: 1} "k1"
});
k2 = null;
wm.get(k2); // undefined