或JavaScript对象声明的符号

Question

I just started to read some JavaScript project. Most of the .js file to start with have an object declared as the following:

window.Example || {
bleh: "123";
blah: "ref"
}

What does the || symbol do here?

Answer 1

Objects in Javascript are truthy, so that expression evaluates to either window.Example or the default object if window.Example is falsy (or undefined). Example:

var x = window.Example || {foo: 'bar'};
// x = {foo: 'bar'}, since window.Example is undefined

window.Example = {test: 1};

var y = window.Example || {foo: 'bar'};
// y = {test: 1}, since window.Example is truthy (all objects are truthy)

Read this article for a good explanation on truthy/falsy and short-circuit evaluation.

Answer 2

The || operator in JavaScript is like the "or" operator in other C-like languages, but it's distinctly different. It really means:

1. Evaluate the subexpresson to the left.
2. If that value, when coerced to boolean, is true , then that subexpression's value ( before coercion to boolean) is the value of the || expression
3. Else evaluate the right-hand subexpression and yield its value as the value of the || expression.

Thus it's used idiomatically to initialize something that might already be initialized:

var something = window.something || defaultValue;

just means, "check to see if "something" is a property of the window object with a truthy value, and if it's not, then set it to defaultValue."

Answer 3

There's an important bit missing from that code - the variable declaration:

var something = window.Example || {
    bleh: "123",
    blah: "ref"
}

This roughly translates to "set something to window.Example , unless it doesn't exist, then set it to this new object".