Javascript: variable = variable2 = variable3

Question

I don't know how to describe this, but this doesn't error out in Javascript... but it doesn't mean its a good idea and that all versions of Javascript except it (or not).

Thoughts?

var r = 'r';
var t = 't';
var s = 's';

s = r = t;

// s = 't'
// r = 't'
// t = 't'

It doesn't seem standard and it may be harder for a developer to follow, but are there really any use cases for this?

related question - http://stackoverflow.com/questions/1758576/multiple-left-hand-assignment-with-javascript — IrkenInvader, Jan 14 '16 at 04:43
It doesn't seem standard and it may be harder for a developer to follow, but are there really any use cases for this? — kungfuu72, Jan 14 '16 at 04:43
Nothing non-standard whatsoever, See example in https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Assignment_Operators — PM 77-1, Jan 14 '16 at 04:46
This is standard practice... Even C accepts this... Equivalent to `s = (r = t)` — Derek 朕會功夫, Jan 14 '16 at 04:50
This is part of the spec (so it is "standard"?): http://www.ecma-international.org/ecma-262/6.0/index.html#sec-assignment-operators . If you come across an environment where this doesn't work, it's a bug. I wonder if you are basically asking why an assignment is an expression and not a statement (in which case `x = y = z` would be a syntax error)? — Felix Kling, Jan 14 '16 at 04:54

score 1 · Answer 1 · answered Jan 14 '16 at 04:50

1

This works because the assignment of r=t returns the assigned value. So yes you can infinitely assign values based on return values.

answered Jan 14 '16 at 04:50

dovidweisz

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score 1 · Answer 2 · answered Jan 14 '16 at 04:57

It is a fairly standard practice to chain assignments together. It even has a name called "chain assignment". I don't know where you get the idea of "this is bad coding" from, but this is definitely a normal thing to see. This works because assignments return the assigned value.

This is commonly used for initializing variables:

var a, b, c;
a = b = c = 5;  //one use case
//same as
a = (b = (c = 5)));

It doesn't seem standard and it may be harder for a developer to follow.

If a developer cannot understand this expression, I don't know what to say.

Robert Moskal · Answer 3 · 2016-01-14T04:57:19.463

I use it all the time in my modules to create a local shortcut to functions (for use within the module):

var doSomething = module.exports.doSomething = ()=>{};

It's nice the way javascript returns values. In languages like python you have to do this sort of thing way to often:

def do_something(ctx):
    ctx.update({
        'entry': entry,
        'section': 'select_category'
    })
    do_something_else(ctx)

instead of

def do_something(ctx):
    do_something_else(ctx.update({
        'entry': entry,
        'section': 'select_category'
    })

because the update method doesn't return anything.

Javascript: variable = variable2 = variable3

3 Answers3