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TypeScript for ... of with index / key?

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How to use index signature TypeScript?

Index signature syntax The syntax of an index signature is pretty simple and looks similar to the syntax of a property, but with one difference. Instead of the property name, you simply write the type of the key inside the square brackets: { [key: KeyType]: ValueType } .

Why we need index signature in TypeScript?

In typescript, Index Signature identifies key type for indexing of an object. Everytime an object in typescript is created and indexing is expected on that object then developers must specify Index Signature .

What is index type in TypeScript?

The indexing type is itself a type, so we can use unions, keyof , or other types entirely: type I1 = Person ["age" | "name"]; type I1 = string | number. type I2 = Person [keyof Person ]; type I2 = string | number | boolean.

What is a signature in TypeScript?

The index signature is a fitting way to handle objects with properties we know nothing about. Its syntax describes a regular property, but instead of writing a standard property name, we define the type of keys and the properties.


.forEach already has this ability:

const someArray = [9, 2, 5];
someArray.forEach((value, index) => {
    console.log(index); // 0, 1, 2
    console.log(value); // 9, 2, 5
});

But if you want the abilities of for...of, then you can map the array to the index and value:

for (const { index, value } of someArray.map((value, index) => ({ index, value }))) {
    console.log(index); // 0, 1, 2
    console.log(value); // 9, 2, 5
}

That's a little long, so it may help to put it in a reusable function:

function toEntries<T>(a: T[]) {
    return a.map((value, index) => [index, value] as const);
}

for (const [index, value] of toEntries(someArray)) {
    // ..etc..
}

Iterable Version

This will work when targeting ES3 or ES5 if you compile with the --downlevelIteration compiler option.

function* toEntries<T>(values: T[] | IterableIterator<T>) {
    let index = 0;
    for (const value of values) {
        yield [index, value] as const;
        index++;
    }
}

Array.prototype.entries() - ES6+

If you are able to target ES6+ environments then you can use the .entries() method as outlined in Arnavion's answer.


https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/entries

for (var [key, item] of someArray.entries()) { ... }

In TS this requires targeting ES2015 since it requires the runtime to support iterators, which ES5 runtimes don't. You can of course use something like Babel to make the output work on ES5 runtimes.


"Old school javascript" to the rescue (for those who aren't familiar/in love of functional programming)

for (let i = 0; i < someArray.length ; i++) {
  let item = someArray[i];
}

You can use the for..in TypeScript operator to access the index when dealing with collections.

var test = [7,8,9];
for (var i in test) {
   console.log(i + ': ' + test[i]);
} 

Output:

 0: 7
 1: 8
 2: 9

See Demo


Or another old school solution:

var someArray = [9, 2, 5];
let i = 0;
for (var item of someArray) {
    console.log(item); // 9,2,5
    i++;
}