What happens to JavaScript literals (strings, numbers) that are not bound (aka assigned) to a variable ?
// A ment
"Practically, also a ment"
var assigned = "something"
53
423.0022
NaN
"does it impact performance"
// or is it treated just like a ment?
The browser appears to ignore them, but I couldn't find a specific rule in the spec
What happens to JavaScript literals (strings, numbers) that are not bound (aka assigned) to a variable ?
// A ment
"Practically, also a ment"
var assigned = "something"
53
423.0022
NaN
"does it impact performance"
// or is it treated just like a ment?
The browser appears to ignore them, but I couldn't find a specific rule in the spec
Share Improve this question edited Jun 30, 2017 at 8:01 Christoph asked Jun 29, 2017 at 20:10 ChristophChristoph 1475 bronze badges 3- 1 For reference: Expression Statement – Mike Cluck Commented Jun 29, 2017 at 20:14
- I don't follow ES3.1, but must be added the strict mode (it's detected through a String literal inside an expression statement by the way) – user5066707 Commented Jun 29, 2017 at 20:14
- It's likely that real world JS engines optimise away "unused" expressions that have no side effects. – joews Commented Jun 29, 2017 at 20:19
5 Answers
Reset to default 8These are "expression statements". Such expressions are evaluated, but since they are not assigned, their value is not stored. JavaScript engines are likely to detect those that have no side effects, and eliminate them as if they were never there.
But still at least one of those has an effect:
"use strict";
This has the meaning of a JavaScript directive
From the EcmaScript specification:
A Use Strict Directive is an ExpressionStatement in a Directive Prologue whose StringLiteral is either the exact code unit sequences
"use strict"or'use strict'.
Also note that other string literals may have a special meaning when used in the directive prologue:
Implementations may define implementation specific meanings for ExpressionStatement productions which are not a Use Strict Directive and which occur in a Directive Prologue.
It's just an expression statement that evaluates - without side effects - to a value that is discarded.
You can see it's the result of the statement if you try this in eval or a REPL.
The values are stored in memory, until they get cleaned up by the Garbage collector, since nothing is pointing to that place in memory.
Since the value is not being stored (i.e. no space is being allocated to hold your literal), it is more or less being ignored by the browser, and also no overhead for the garbage collector. At least the function of your code does not change. Your javascript file will of course be a tiny bit larger because it contains more (useless) text.
It appears to treat them as variables being inserted to nothing. It treats them as if they don't exist anyway. Try it:
function myFunction() {
"ment"
//Or ment
alert("hello");
}<html>
<body>
<button onclick="myFunction()">Call Javascript</button>
</body>
</html>As you can see, nothing happens when the empty string is there. This is an interesting question, so I would be very interested in hearing any other results.