I was reading Chapter 2: this
All Makes Sense Now! from You Don't Know JS , and decided to do this experiment.
I have this simple enough script foo.js
:
var a = 'foo';
var output;
// lets find a way to output strings in both
// Chrome and Node.js
if (typeof alert === 'undefined') {
output = console.log;
} else {
output = alert;
}
function getA() {
return this.a;
}
var foo = getA();
output(foo);
I am expecting following things when getA()
is called:
getA
is in global scope, getA()
will be bound to global object. var a
is declared in global scope, I take it that global object will have a property named a
, and this property is same as the variable a
. this.a
to refer to variable a
. output(foo)
to print the string foo
. However, when run in Node.js (non-strict mode), this is the output:
$ node foo.js
undefined
Then I included the same script in a simple HTML page, and loaded it in chrome.
<html>
<head>
<script src="foo.js" type="text/javascript"></script>
</head>
<body>
</body>
</html>
Chrome alert
s the string foo
, just as expected.
Why does the output of Chrome differ from Node.js?
Since the call site of getA is in global scope, getA() will be bound to global object.
This is a misunderstanding of the this
binding rules from my book. The call site's location (aka "in global scope") is entirely irrelevant. It's the manner in which the call is made, and only that.
It's not where getA()
happens that matters, but that getA()
is a plain normal function call. THAT is what determines that you'll get the global object bound to the this
for that call.
The other answers here are correct... the scope your node main program runs in is actually a module (function wrapped), not a real global scope.
NodeJS behaves differently than browsers. The top-level scope is not the global scope, it's the scope within that file or module. Drop the "var" and your code will work (a will become truly global) in a node environment and it will console.log the string 'foo'.
See the following page for a full reference: http://nodejs.org/api/globals.html
OR
Since the call site of getA is in global scope, getA() will be bound to global object.
no, that's not true for node - your script is wrapped into a function here so your example is actually this code:
(function (exports, require, module, __filename, __dirname) {
var a = 'foo';
var output;
// lets find a way to output strings in both
// Chrome and Node.js
if (typeof alert === 'undefined') {
output = console.log;
} else {
output = alert;
}
function getA() {
return this.a;
}
var foo = getA();
output(foo);
})(exports, require, module, 'file.js', '/dir/name');
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