Common programming knowledge in javascript_basic knowledge

1.null and undefined

* Obtain a property from an object. If neither the object nor the objects in its prototype chain have the property, the value of the property will be undefined.
* If a function does not explicitly return a value to its caller through return, its return value is undefined. There is a special case when using new.
* A function in JavaScript can declare any number of formal parameters. When the function is actually called, if the number of parameters passed in is less than the declared formal parameters, the value of the extra formal parameters will be undefined.
* null is an empty object, pay attention to the difference from empty objects ({}).
To put it simply, for all variables, as long as the initial value has not been specified after declaration, then it is undefined. If the Object type is used to express the concept of null reference, then it is represented by null.

2.if expression

* Null is always false (false)
* Undefined is always false (false)
* Number 0, -0 or NaN is false, other values ​​are true
* String When the string is empty, it is false, and other values ​​​​are true
* Object is always true (true)

3.Array

Arrays in JavaScript are very different from those in common programming languages, such as Java or C/C. An object in JavaScript is an unordered associative array, and Array is implemented by taking advantage of this feature of objects in JavaScript. In JavaScript, Array is actually an object, but its property names are integers. There are many additional properties (such as length) and methods (such as splice) to conveniently operate arrays.

4.new operator

JavaScript does not have the concept of a class in Java, but uses a constructor to create objects. New objects can be created using constructors in new expressions. Objects created by a constructor have an implicit reference to the constructor's prototype.

5.prototype

Prototype is the core concept of javascript prototypal inheritance. You must have seen the usage of Array.prototype.push.call() in a certain javascript class library, so in the final analysis, prototype is an object. We can add some useful methods to the native class through prototype, and we can also implement inheritance through prototype. If you are more interested in prototype, you can access the prototype chain of the specified object through __proto__ in ff.

6.scope chain

Execution context is an abstract concept used in the ECMAScript specification to describe the execution of JavaScript code. All JavaScript code runs within an execution context. When function is called in the current execution context, a new execution context will be entered. When the function call ends, it will return to the original execution context. If an exception is thrown during a function call and is not caught, it may exit from multiple execution contexts. During the function calling process, other functions may also be called to enter a new execution context. This forms an execution context stack.

Note: If you are more interested in the scope chain, you can access the function's scope chain through the __parent__ attribute in ff. Unfortunately, ff's js engine SpiderMonkey does not have perfect support for this attribute, and internal functions appear. An error will occur, so it is recommended to use Rhino (http://developer.mozilla.org/en/docs/Rhino).

Note: What needs to be mentioned here is the impact of function expressions and function declarations on the scope chain.

7.closure

Closure is also a commonly used feature of js. Generally, when we execute a function in java, all internal variables will be recycled. However, in javascript, we can use certain methods to make the executed function The internal variables still exist and are accessible, thus forming a closure.

Of course, everything has two sides. While closures bring us benefits, they can also cause a lot of troubles, such as memory leaks if we are not careful, so we need to apply them reasonably. these technologies. If you want to learn closures in depth, it is recommended to check out several js libraries (prototype, jquery, mootools), which contain some very classic closure applications, such as the bind method, etc., which I will not go into details here.