JavaScript expressions and unary operators

First, expression:
An expression is a "phrase" in ECMAScript, and the interpreter converts it to a value by calculation. The simplest expression is the literal or variable name. For example:

 5.96 //  numeric literal  ' Lee ' //  true  //  Boolean literal  null  //  null literal /java///  regular expression literal  {x:1, y:2} //  [
     
      //
       array literal, array expression  function  (n) {return  x+y;} //  box //  variable  

Of course, you can also create complex expressions by merging simple expressions. Like what:

// An expression of an addition operation typeof // to view an expression of a data type // Logical Operation Expressions

　　

Two or one-tuple operator:

An operator that can manipulate only one value is called a unary operator.

1, increment + + and decrement--

The difference between front and rear:

In the absence of an assignment operation, the front and back are the same.

In an assignment operation, if the increment or decrement operator is pre-placed, then the predecessor operator accumulates or decrements the assignment first, and if it is the post operator, the value is then incremented or subtracted.

var box = +; // forward increment, add box 1, equivalent to box = box+1 // decrease the front, reduce box by a 1, equivalent to box = Box-1 // post-increment, ibid. // post-decrement, ibid.

var box = +; var // an age value of 101,box is added to 101 and then assigned to age. var //  Box is assigned to height before it is self-increasing.

     var box = +;       var height =--box      alert (height); // The result is 99, which is first self-subtraction and then assigned to height.        var box = +;       var age = box--;      alert (age); // The result is that 100,box is first copied to age and then self-reducing.

2. Other types of rules that apply unary operators

 　   varbox = ' 89 '; box++;//90, numeric strings are automatically converted to numeric values 　　 varbox = ' AB '; box++;//nan, String contains non-numeric to Nan 　　 varbox =false; ++box;//The 1,false turns into a value of 0, and the summation is 1. 　　 varbox = 2.3; ++box;//3.3, add 1 directly 　　 varbox = {//2, no toString or valueOf is set to NaNTostring:function() {return1; }　　}; Box++;

var box = {};alert (box+ +); // NaN

3, plus and minus operators:

The add operation rules are as follows:

 　　　　 varbox = 100; +box;//100, no effect on the value 　　　　 varbox = ' 89 '; +box;//89, numeric string converted to numeric value 　　　　 varbox = ' AB '; +box;//nan, String contains non-numeric to Nan 　　　　 varbox =false; +box;//0, the Boolean value is converted to the corresponding value 　　　　 varbox = 2.3; +box;//2.3, no change 　　　　 varbox = {//1, do not set toString or valueOf is NaNTostring:function() {return1; }　　　　}; +box;

The following are the rules for reducing operations:

varbox = 100; -box;//-100, for the value, the direct change negative 　　　　 varbox = ' 89 '; -box;//-89, numeric string converted to numeric value 　　　　 varbox = ' AB '; -box;//nan, String contains non-numeric to Nan 　　　　 varbox =false; -box;//0, the Boolean value is converted to the corresponding value 　　　　 varbox = 2.3; -box;//-2.3, no change 　　　　 varbox = {//-1, does not set toString or valueOf is NaNTostring:function() {return1; }　　　　}; -box;

The addition and subtraction operators are generally used for arithmetic operations, as well as for type conversions above.

JavaScript expressions and unary operators