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About

This dependency reimplements the for...in statement which is used to iterate over objects.

In fact, it even uses the for...in statement for its re-implemention, this is the whole source code:

/*!
 * for-in <https://github.com/jonschlinkert/for-in>
 *
 * Copyright (c) 2014-2017, Jon Schlinkert.
 * Released under the MIT License.
 */

"use strict";

module.exports = function forIn(obj, fn, thisArg) {
    for (var key in obj) {
        if (fn.call(thisArg, obj[key], key, obj) === false) {
            break;
        }
    }
};

for...in usage

for...in is part of the ECMAScript standard and is widely supported in modern browsers and Node.js.

const obj = {
    foo: "bar",
    baz: "qux"
};

const keys = [];
const values = [];

for (const key in obj) {
    keys.push(key);
    values.push(obj[key]);
}

console.log(keys); // ["foo", "baz"]
console.log(values); // ["bar", "qux"]

To replicate the early exit behavior of the for...in statement, you can use the break statement:

const obj = {
    foo: "bar",
    baz: "qux"
};

const keys = [];
const values = [];

for (const key in obj) {
    // early exit
    if (key === "baz") {
        break;
    }

    keys.push(key);
    values.push(obj[key]);
}

console.log(keys); // ["foo"]
console.log(values); // ["bar"]

However for...in has some caveats, see this article for more information.

You are more likely to want to use for...of instead, see this article for more information.