forEach 接受一个数组和一个回调

Question

I need help for this problem because I don't understand it :(. Please help!

The function forEach takes an array and a callback. It runs the callback on each element of the array - and that's it!

var alphabet = '';
var letters = ['a', 'b', 'c', 'd'];
forEach(letters, function(char) {
  alphabet += char;
});
console.log(alphabet);   //prints 'abcd'


**(underneath is is where I edit)**



// Challenge 4
function forEach(array, callback) {

}

// see for yourself if your forEach works!

Answer 1

I do not know exactly what you want to achieve, but your code is not idiomatic JavaScript...

The core JavaScript language already provides a forEach method that you can use directly. Please read the following page: Array.prototype.forEach()

When you use forEach , your code should look like this:

var alphabet = '',
    letters = ['a', 'b', 'c', 'd'];

letters.forEach(function (letter) {
  alphabet += letter;
});

console.log(alphabet);

But if you are concerned about performance leaks, you may want to use a traditional for loop with cached length instead.

var alphabet = '',
    letters = ['a', 'b', 'c', 'd'];

for(var i = 0, len = letters.length; i < len; i++) {
  alphabet += letters[i];
}

console.log(alphabet);

Thanks to ES6, you can also use the for...of loop on iterable objects like arrays. This is the recommended instruction in modern JavaScript applications:

let alphabet = '',
    letters = ['a', 'b', 'c', 'd'];

for (const letter of letters) {
  alphabet += letter;
}

console.log(alphabet);

But since developers are lazy sometimes, the best answer is probably this one:

var letters = ['a', 'b', 'c', 'd'],
    alphabet = letters.join('');

console.log(alphabet);