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Am not new to JS or its syntax, but sometimes, the semantics of the language has me stumped at times. At work today, a colleague mentioned this:
var a = b = []; is not the same as
var a = [], b = []; or
var a = []; var b = []; since the first version actually assigns the reference to an empty array to a and b. I couldn't quite accept this as true, but I'm not sure. What do you all think?
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Chained assignments are used as a shortcut when you want to bind several variables to the same value. Copy x = y = somefunction() which is the same as this: Copy y = somefunction() x = y. Note that the preceding statements may not be the same as.
“JavaScript allows multiple left-hand assignments” Well, if you're not familiar, in JavaScript you can write the variable assignment expressions like this. var a = b = c = d = 10; The assignment expressions are always evaluated from right-to-left.
The simple assignment operator ( = ) is used to assign a value to a variable. The assignment operation evaluates to the assigned value. Chaining the assignment operator is possible in order to assign a single value to multiple variables.
Yes, they're not the same. var a = b = [] is equivalent to
var a; b = []; a = b; Not only do both a and b get assigned the same value (a reference to the same empty array), b is not declared at all. In strict mode in ECMAScript 5 and later, this will throw a ReferenceError; otherwise, unless there is already a variable b in scope, b is silently created as a property of the global object and acts similarly to a global variable, wherever the code is, even inside a function. Which is not good.
You can see this quite easily:
(function() { var a = b = []; })(); console.log(b); // Shows [] If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
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