JS Basic knowledge 1

1. Components

• DOM: The Document Object model, the DOM tree, itself provides a set of methods (APIs) for manipulating page elements.

• BOM (Browser object model): Browser object models core object (Window loacation).

• ECMAScript: Specify JS core syntax, JS statements, keywords, reserved words

2. Two types of data

• Basic data type (simple data type):

  string literal type

  Number value

  Boolean Boolean type (True | false)

  Undefined not defined

  Null empty (object)

• Complex data types (composite data types):

  Object type

  Array type

  funcation function type

  Date type

  Math Object

  String Object

  Number Object

  Boolean Object ...

3. Assignment, equals and congruent description

Assignment (=): Assigns a variable to a specific data;

equals (= =): Only when the comparison determines whether the value is equal;

Congruent (= = =): Comparison of the time not only to compare the data also need to compare types;

Not equal to (! =)

Not all equals (!==)

var str1 = "Let Me fall tears, not only your gentleness"; String type

4. Relational operators

Return value: Boolean type, either True or false, typically used in a judgment statement.

Greater than >

Less than <

Greater than or equal to >=

Less than or equal to <=

5. Logical operators

• Logical NON (!) Take counter

• Logical OR

  Syntax: expression 1 | | Expression 2

  Result: The expression 1, if the expression 1 is true, then return expression 1, if the value of the expression 1 is false, then return the expression 2;

  var Resulta = 3 | | 2; 3

• Logic and (&&)

  Syntax: expression 1 && expression 2

  Result: If expression 1 is true, return expression 2, return expression 1 if expression 1 is false

  var Resulta = 3 && 2; 2

6. Value types and reference types

• Value type:

  String, number, Boolean, undefined

• Reference type:

  Object type

  Difference: A value type stores a specific value, and a reference type stores a pointer to a specific address (reference);

  Value type of data

• Value types and assignment of reference types

  ① Value type assignment:

  Copy the stored value (specific data) from the right variable to a variable on the left

  ② Reference type Assignment:

  The right variable stores the stored value (pointing to the address of the specific data) and copies a copy to the left variable

  Difference:

  Value type assignment is the replication of simple data, they are kept in a relatively independent space, they do not affect each other, the reference type assignment modifies the properties of an object and affects the properties of another object;

  Value type assignment:

• The use of value types and reference types in functions

  Related terms within a function

  Arguments (actual parameters):

  Formal parameter (formal argument): A placeholder variable that has no value when the parameter is not called

  function invocation: By default, the value of the argument is assigned to the parameter

  Difference: value types are arguments to functions, arguments, and parameters their data are independent of each other. A reference type is an argument to a function, and a formal parameter and an argument share the same data, and modifying one of the objects affects the other.

JS Basic knowledge 1