在NodeJS中var export = module.exports = {}

Question

What I get from previous posts and articles is that the exports object is located on the global object. I came across this code which confused me:

let blue = 'blue'
let red = 'red'

var exports = module.exports = {
    red,
    blue
};

This code sets module.exports to a variable called exports which then gets set to an object which gets exported.

I am confused however by this syntax:

Example1:

var exports = module.exports = {}

How does this work exactly? Because normally in JS you can't assing a variable two times. For instance this gives an error:

Example2:

let foo = 5 = 4;

How does the code in example 1 give no error while the code in example 2 does?

Answer 1

let foo = 5 = 4;

Cause its parsed from right to left:

let foo = (5 = 4);

And 5 is not a variable, so you cant assign stuff to it. However it works with an identifier:

let bar;
let foo = bar = 5;

Answer 2

Your interpretation of what the line is doing is incorrect.

This code sets module.exports to a variable called exports which then gets set to an object which gets exported.

What is actually happening is that the value { red, blue } is being assigned to module.exports , and then that same value ( { red, blue } ) is being assigned to exports .

In JavaScript and other languages with similar syntax (C, C++, C#, Java) someAssignableThing = someValue is treated as an expression , and you can use a = b as a sub-portion of other expressions and chain as many together as you want.

As an expression someAssignableThing = someValue equates to "assign someValue to someAssignableThing and evaluate to the value someValue ".

So the statement:

a = b = c = d = e = 5;

would assign the value 5 to a , b , c , d , and e .

It is a syntax error to have something on the left side of the = that cannot be assigned a value and that's why you get an error in the second case (you cannot assign a value to 5 ).