Operator:
- Assignment operator = + = - = * =/=
- Arithmetic operator + - * / divide by% (remainder) the remainder is positive or negative, and the positive or negative is determined by the divisor
- Relational operator = = = ====== !== > = < = > < < get Boolean types
- = = as like as two peas, the same value is not considered.
- Logical operators & & and | or! Reverse
- &&As long as one is false, the result is false | as long as one is true, the result is true
<script>
// Assign the value of a to a
var a=2;
// a=a+2; Abbreviated as a+=2;
// a+=2;
// a=a-2, abbreviated as a-=2;
// a=a/2, abbreviated as a/=2;
// a=a*2, abbreviated as a*=2;
console.log(10/3);
console.log(10%-3);
console.log(-a);
console.log(1===1);
console.log(1!='1');
a=800;
// Cast when the code performs Boolean and numeric type comparison step by step
// console.log(10<a<100)
// In the case of pure numerical and or non. The result is also a numerical value.
// console.log(a>10&&a<100);
// The code executes from left to right
console.log(2&&3&&4&&0&&3)
console.log(0||0||4||4||3)
</script>Process control statement IF
Format I
if (conditional){
When the condition is true, execute the code.
}
Format II
if (conditional){
When the condition is true, execute the code.
}else{
When the condition is false, execute the code.
}
<script>
var a=prompt('Please enter salary');
a=parseInt(a);
//if(age<18){
// Popup
// alert('older ')
// }
if(age<18){
alert('Not old enough')
}else{
alert('Internet cafe')
}
//0 is false undefined is false
// All strings are true
// if(!1){
// console.log('hahah')
// }
</script>Multi condition judgment: (if statement has jump)
If (condition 1){
When condition 1 is true, execute the statement and jump out directly
} else} if (condition 2){
When condition 2 is true, execute
} else} if (condition 3){
When condition 3 is true, execute
}.....
else{
If the above conditions are not met, it shall be implemented
}
<script>
if(a<=36000){
// For different variable types, if splicing is required, use+
alert('You need to pay tax'+a*0.03);
}else if(a<=144000){
var sum=36000*0.03+(a-36000)*0.1;
alert('You need to pay tax'+sum)
}else if(a<=300000){
var sum=36000*0.03+(144000-36000)*0.1+(a-144000)*0.2;
alert('You need to pay tax'+sum)
}else if(a<=420000){
var sum=36000*0.03+(144000-36000)*0.1+(300000-144000)*0.2+(a-144000)*0.2;
alert('You need to pay tax'+sum)
}
</script>loop
Loop: the code used to implement traversal is sequential
- Step 1: execute a, initialization
- Step 2: judge whether it complies with b. if it does, execute step 3, otherwise it will jump out of the loop
- Step 3: execute the loop body and execute c. Repeat step 2
<script>
//for(a initialization; b condition; c condition / times of continuation){
// Circulatory body
// }
for(var a=1;a<=100;a++){
console.log(a);
}
</script>
function
Function: encapsulation, which encapsulates repeated code. Generally, these codes have special purposes
Function definition: the naming rules for function names are the same as those for variable names
<script>
// Function function name (){
// Function body
// }
// Function name ()
// Define function
function fn(){
console.log('zsh')
console.log('sh')
}
// Using a function is equivalent to executing the code of the function body
fn()
</script>
Parametric function:
When defining a function, the parameters are called formal parameters, which is equivalent to declaring variables
When using a function, the parameters are called arguments, which is equivalent to assigning values to variables
<script>
// Parameters when defining a function, called formal parameters, are equivalent to declaring variables
function fn(a){
console.log(a)
}
fn(2)
fn(3)
</script>Function with return value:
Use this function to get the contents after return
<script>
function fn(a,b){
// Use this function to get the contents after return
return a+b;
}
var c=fn(1,2)
document.write(c)
function mul(a,c){
return a*c
}
mul(12,2)
</script>Scope of variable
The variables declared in js only have the scope of the function.
Scope of the declared variable. The scope of action of the {variable of the function nearest to the variable is}.
<script>
function fn(){
var a=10;
console.log(a)
}
fn()
var b;
console.log(b);
//No error will be reported and undefined will be printed
</script>Variable will promote the declaration to the front of the whole scope, and the assignment is still in the original position
<script>
console.log(a);
var a=100;
// Equivalent to
// var a;
// console.log(a)
// a=100;
</script>Global variables and local variables exist at the same time, and local variables take effect
<script>
var a=1;
function fn(){
console.log(a);
var a='hello';
// var a;
console.log(a);
// a='hello'
}
fn();
</script>For undeclared variables, the scope takes effect after assignment (not recommended)
<script>
a=100;
console.log(a);
console.log(b);
b=100;
</script>
Control of DOM elements
Array: an array stores a set of data
Get an element of the array: the array name [subscript] subscript starts from 0
Get the length of the array: array name length;
Get the tag by ID Name: document Getelementbyid ('id name ');
Event: get tag on + event type (click)=function() {executed code};
Modify content: get label innerHTML = 'modified content'
Generate random number: math Random() generates random numbers between 0 and 1, including 0 and excluding 1
Timer: setInterval(function() {code executed every other period of time}, time ms)
Stop timer: clearinterval (timer name)
<body>
<button id="btn">Button</button>
<script>
var list=['a','b','c','d'];
console.log(list[2])
// Array length array name length
console.log(list.length)
var tag=document.getElementById('btn');
tag.onclick=function(){
tag.innerHTML='It's too late';
console.log(Math.random())
// Timers can be superimposed
setInterval(function(){
console.log(Math.random())
},1000)
}
</script>
</body>Lucky draw cases:
<body>
<!-- id The name is similar to our ID number. It is unique. -->
<button id="start">start</button>
<button id="end">end</button>
<h4 id="txt">Let's start the lottery</h4>
<script>
var start=document.getElementById('start');
var end=document.getElementById('end');
var txt=document.getElementById('txt');
var list=['ddd','xxx','vvvv','aaa','bbb','CCC','XXX'];
var timer;
// Click start
start.onclick=function(){
clearInterval(timer)
timer= setInterval(function(){
// The random number of subscripts is generated
var n=parseInt(Math.random()*list.length);
console.log(n);
// Modify html content
txt.innerHTML=list[n];
},50)
}
// Click end
end.onclick=function(){
// Stop timer clearinterval (timer name)
clearInterval(timer)；
}
</script>
</body>