CODEWITHSUNDEEP
javascriptvariablesreference
Richard
Richard

Reputation: 5101

Why is this method of variable assignment causing issues?

How does this style of variable assignment (it's inside an object so the "this" variable refers to said object)):

var prop       = this.properties;
var properties = this.properties;

Differ from this?

var prop = properties = this.properties;

When I changed to the latter it caused issues. Is it a referencing issue?

Upvotes: 2

Views: 136

Answers (1)

T.J. Crowder
T.J. Crowder

Reputation: 1074355

The difference is that with the second one, you're trying to assign to a properties variable that you haven't declared. The var only applies to prop, not properties because properties is on the right-hand side of the = after prop, so it's part of the initializer for prop. Here's how it's grouped:

var prop = (properties = this.properties);
//         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^−−−− initializer for `prop`

In loose mode, that means you're falling prey to what I call The Horror of Implicit Globals — it creates a global called properties. In strict mode, assigning to an undeclared identifier is the error it always should have been. (All new code should be written using strict mode.)

You can declare multiple variables with var, etc., but you can't assign to them all using the same value source (without repeating it).

You could do:

var prop, properties;
prop = properties = this.properties;

if you liked. Or alternatively:

var prop = this.properties, properties = prop;

That works because the initializers in a var (or let or const) with multiple declarations are executed in source code order, left-to-right, so prop = this.properties happens before properties = prop does.

Side note: var shouldn't be used in new code, use let or const. (If you have to support obsolete JavaScript engines, you can transpile modern code to ES5.)

Upvotes: 4

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