What does "var FOO = FOO || {}" (assign a variable or an empty object) mean in JavaScript?
In JavaScript, you may encounter code snippets like:
var FOO = FOO || {};
FOO.Bar = …;
where || {} seems enigmatic. This construction has a specific purpose and is commonly used at the beginning of JavaScript source files.
Understanding the Namespace Pattern
var FOO = FOO || {}; establishes a namespace object named FOO. This pattern is particularly useful when dealing with multiple JavaScript files that need to share and encapsulate functionality without polluting the global object.
How it Works
The || operator acts as a conditional assignment. It first checks if FOO already exists as a variable. If it does, FOO is assigned its existing value. If it doesn't, FOO is assigned the default value of {}, an empty object. This guarantees that FOO will always be an object.
Benefits of Namespace Objects
Employing namespace objects provides several advantages:
- Modularization: Allows for grouping related functionality within named objects.
- Scope Control: Helps prevent variable name conflicts between different files.
- Asynchronous Loading: Supports asynchronous loading of JavaScript files while maintaining namespace consistency.
Example
Consider two files that share the same namespace:
// File 1
var MY_NAMESPACE = MY_NAMESPACE || {};
MY_NAMESPACE.func1 = {
// ...
};
// File 2
var MY_NAMESPACE = MY_NAMESPACE || {};
MY_NAMESPACE.func2 = {
// ...
};
Regardless of the loading order, MY_NAMESPACE.func1 and MY_NAMESPACE.func2 will be accessible within the shared namespace object. This pattern ensures the proper initialization and organization of functionality across multiple JavaScript files.
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