Javascript forEach Console.log without lambda like syntax-CodePudding

I am going through a JS Book (Modern Javascript for the Impatient) and came across this example, which had a different output than what I was expecting.

const numbers = [1, 2, 3]
numbers.forEach(console.log)

1 0 [ 1, 2, 3 ]
2 1 [ 1, 2, 3 ]
3 2 [ 1, 2, 3 ]

When I use a lambda expression like syntax, I see the right (expected) result.

const numbers = [1, 2, 3]
numbers.forEach(number => console.log(number))

1
2
3

How is the expression being evaluated in the first case? Is the lambda expression syntax always expected?

CodePudding user response：

It has nothing to do with arrow function expression (lambda) syntax.

console.log() accepts a series of arguments.

The callback signature for Array.prototype.forEach() is:

forEach((element, index, array) => { /* … */ })

So, for every invocation of the callback (console.log in your example), the series of arguments is displayed in the console.

CodePudding user response：

foreach sends in more than just the value. It also sends in the index and original array as arguments 2 and 3. These leak into console.log using the non lambda syntax.