21 basic knowledge points of JavaScript_Basic knowledge

1. JavaScript is case-sensitive;
2. If you declare a variable without writing var, you have declared a global variable; any function that is not a method is a global variable, and this in it points to window;
3. % operator, find the remainder, keep the integer, y=5;x=y%2; then x=1;
4. If you need to connect two or more string variables, please use the operator, the difference in php;
5.Ternary operation: greeting=(visitor==”PRES”)?”Dear President “:”Dear “;
6.indexOf() method, counting from 0, spaces also count One digit;
7.Math.floor() rounds down the integer, and the returned value is less than or equal to the number, Math.floor(-5.9), returns -6;
8. When declaring variables inside the function , be sure to use the var command. If you don’t use it, you actually declared a global variable!
9. When writing a demo, be sure to write JS to the bottom to prevent the JS code from not running as expected due to the html element not being loaded (JS is Interpreted language, interpreting while loading, only the target object can be obtained and manipulated after it is loaded into the DOM tree);
10. The meaning of closure: Closure is actually a nesting of functions, and inner functions can use outer functions. All variables of the outer function, even if the outer function has been executed (so the closure will have performance problems, the outer variables will still exist in the memory after the outer function call is completed);
11. The value of the checkbox is check, not true; the value of cancellation is undefined, not false;
12. Use typeof(flag)===”undefined” to judge that a variable is undefined;
13. Global variables can be accessed inside JS functions (or as Function external variables), this is different from PHP;
14. This in a function always points to the caller, or it can be said: for functions that are not methods, this points to the window; for functions that are methods, this points to the object itself (Pay attention to the closure problem in the method, this points to window);
15. Method of destroying variables: obj = null; delete obj;
16. When judging directly, the following will be converted to false: undefined,null,0,-0,NaN,""(empty string); all other values, including objects and arrays, will be converted to true;
17. Boolean values ​​include a toString() method, which can return true or false string;
18. It can be said that JavaScript has only 6 data types, numbers, strings, Boolean values, null, undefined and objects;
19. There is no block-level scope in JavaScript, instead there are functions Scope (see Rhinoceros Book Sixth Edition P57);
20. JavaScript declaration advance feature: all variables declared in a function are advanced to the top of the function (see Rhinoceros Book Sixth Edition P58);
21. The execution of functions in JS is asynchronous, so pay attention to the value issue.