Question

what’s the difference between Browsers and Node? for instance:

setName.js on Node:

var setName;
setName = function (name) {
    return this.name = name;
};
setName("LuLu");
//LuLu
console.log(name);
//undefined
console.log(this.name);

setName.html in browser:

<script>
    var setName;
    setName = function (name) {
        return this.name = name;
    };
    setName("LuLu");
    //LuLu
    console.log(name);
    //LuLu
    console.log(this.name);
</script>

the the second log is different,why?

Answer 1

Node is a JavaScript engine, not a browser. The specific reason that you see undefined in Node, and Lulu in a browser? Differences in the global namespace:

In browsers, the top-level scope is the global scope. That means that in browsers if you’re in the global scope var something will define a global variable. In Node this is different. The top-level scope is not the global scope; var something inside a Node module will be local to that module.

In the browser, this is a reference to the window object — the browser’s global namespace — for all functions which are invoked unattached to an object (e.g. not like foo.bar()). In Node, this is simply not a reference to the global namespace.

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N.B. console.log(this.name) in a Node interpreter will print Lulu, not undefined. That’s because, in the REPL only,

> this === global
true

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Further reading @ How To Node: What is “this?”

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Okay, one more edit as prompted by @Šime Vidas’ comment regarding this in ES5 strict mode:

• In the global context (outside of any function), this refers to the global object, whether in strict mode or not.
• When the this keyword occurs inside a function, its value depends on how the function is called.
• When a function is called as a method of an object, its this is set to the object the method is called on.

More interesting reading courtesy of Juriy Zaytsev (aka @kangax) in one of his blog posts.