JS Authoritative Guide---study notes 02

1, JS only function scope, there is no block-level scope of the concept;

It has a feature-declaration in advance: variables declared in different positions in the same function are declared at the beginning of the function, and the assignment is performed at the original location;

2, the Declaration of global variables, equivalent to define a global object of a property;

1) If you declare a global variable using VAR, this property is not configurable. Which means you can't delete.

2) If you do not use the Var declaration, you can delete;

Example: var S1 = "S1"; Non-configurable global variables

S2 = "S2"; Configurable global variables

Delete S2; The delete operation can be performed and the variable will be deleted

3. ★ Scope Chain

4. Original expression: the smallest unit of an expression.

Contains: constants, direct quantities, keywords, variables;

PS: The direct amount is the constant value in the program that appears directly. 1.23 Digital Direct Volume "Hello" string Direct amount

5, the difference between keywords and reserved words: The keyword is JS has been used, has a certain meaning character. A reserved word is a reserved keyword that may not be a keyword now, and may become a keyword later.

6. Object and array initialization expressions are actually creating a new object or arrays. Also known as: Object direct amount, array direct amount, however they are not original expression.

7, array initialization, if there is no value in the middle, can be omitted directly;

var myArray = [,,, 3]; Two values are omitted in the middle and are automatically populated with undefined

var myArray = [1, 2,]; If the end ends with a comma, the undefined will not be populated

8, function definition expression: Also called function direct quantity;

var myFunc = function (x) {return x*x;} The function is to return the square of a number

9. Property access expression: Gets the value of an object property or an array element.

Directly using "point" or [] to get the value

Note: The attribute name cannot be a reserved word, a keyword, a space, or a punctuation mark when accessed with a "point";

10. Call expression: A representation of the calling function or method. such as: alert (' Hello ')

-If the called function returns a value using return, then this value is the value of the entire invocation expression. Otherwise, it's undefined.

JS Authoritative Guide---study notes 02