JavaScript syntax explained: If statement &for loops & Functions

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If statement of the most basic if statement

Structure of the IF statement: (format)

    if{        // 条件为真时,做的事情    }else{        // 条件为假时,做的事情    }

If statements also become "SELECT statement", "conditional judgment statement", we use the following image to explain.

Multi-Branch If statement


    if (条件表达式1{        // 条件1为真时,做的事情    }elseif (条件表达式2{        // 条件1不满足,条件2满足时,做的事情    }elseif (条件表达式3{        // 条件1、2不满足,条件3满足时,做的事情    }else{        // 条件1、2、3都不满足时,做的事情    }

In all of the above statement bodies, only one is executed.

Do a topic:

    根据BMI(身体质量指数)显示一个人的体型。    BMI指数,就是体重、身高的一个计算公式。公式是:    BMI =体重÷身高的平方    比如,老师的体重是81.6公斤,身高是1.71米。    那么老师的BMI就是  81.6 ÷ 1.712     等于 27.906022365856163    过轻:低于18.5    正常:18.5-24.99999999    过重:25-27.9999999    肥胖:28-32    非常肥胖, 高于32    用JavaScript开发一个程序,让用户先输入自己的体重,然后输入自己的身高(弹出两次prompt框)。    计算它的BMI,根据上表,弹出用户的身体情况。比如“过轻” 、 “正常” 、“过重” 、 “肥胖” 、“非常肥胖”。


Notation 1:

        //First step, enter height and weight        varHeight= parsefloat(prompt("Please enter the height, the unit is the meter"));        varWeight= parsefloat(prompt("Please enter weight, unit is kg"));        ///Step two, calculate the BMI index        varBmi=Weight/Math.POW(Height, 2);        //Third step, if statement to judge. Notice the phenomenon of jumping        if(BMI< 18.5){            Alert("Thin");        } Else if(BMI<  -){            Alert("Normal");        } Else if(BMI<  -){            Alert("overweight");        } Else if(BMI<=  +){            Alert("Obesity");        } Else {            Alert("very fat.");        }

Notation 2:

        //First step, enter height and weight        varHeight= parsefloat(prompt("Please enter the height, the unit is the meter"));        varWeight= parsefloat(prompt("Please enter weight, unit is kg"));        ///Step two, calculate the BMI index        varBmi=Weight/Math.POW(Height, 2);        //Third step, if statement to judge. Notice the phenomenon of jumping        if(BMI>  +){            Alert("very fat.");        } Else if(BMI>=  -){            Alert("Obesity");        } Else if(BMI>=  -){            Alert("overweight");        } Else if(BMI>= 18.5){            Alert("Normal")} Else {            Alert("Thin");        }
Nesting of IF statements

Let's use the following example to draw a nested if statement.

    一个加油站为了鼓励车主多加油,所以加的多有优惠。    92号汽油,每升6元;如果大于等于20升,那么每升5.9;    97号汽油,每升7元;如果大于等于30升,那么每升6.95    编写JS程序,用户输入自己的汽油编号,然后输入自己加多少升,弹出价格。

The code is implemented as follows:

        //First step, enter        varBianhao= parseint(prompt("What oil would you like to add?" Fill in 92 or "));        varSheng= parsefloat(prompt("How many liters would you like to add?" "));        //Second step, Judge        if(Bianhao==  the){            //number is 92 time to do things            if(Sheng>=  -){                varPrice=Sheng* 5.9;            } Else {                varPrice=Sheng* 6;            }        } Else if(Bianhao==  the){            //number is 97 time to do things            if(Sheng>=  -){                varPrice=Sheng* 6.95;            } Else {                varPrice=Sheng* 7;            }        } Else {            Alert("Sorry, there's no petrol for this number!" ");        }        Alert("The Price is" +Price;
Several small knowledge points of the IF statement

(1) Else part can be omitted. For example:

        var=10;        if>20){            alert("这个数字大于20");        }

There is no else part, which means there is no "otherwise". If the conditional expression is not satisfied, then nothing is done.

(1) If there is a sentence to be done, the curly braces can be omitted. For example:

        var=2;        if>5alert("这个数字大于5");        alert("哈哈");

The popup content is "haha".

Structure of For loop for loop

For loop Example:

    for (var=1;<=100; i++{        console.log(i);    }

Explanation of the above code:

For loop traversal
    for (var=1;<13;=+4{        console.log(i);    }

The traversal steps of the above code:

程序一运行,将执行var i = 1;这条语句, 所以i的值是1。然后程序会验证一下i < 13是否满足,1<13是真,所以执行一次循环体(就是大括号里面的语句)。执行完循环体之后,会执行i=i+4这条语句,所以i的值,是5。程序会会验证一下i < 13是否满足,5<13是真,所以执行一次循环体(就是大括号里面的语句)。执行完循环体之后,会执行i=i+4这条语句,所以i的值,是9。程序会会验证一下i < 13是否满足,9<13是真,所以执行一次循环体(就是大括号里面的语句)。执行完循环体之后,会执行i=i+4这条语句,所以i的值,是13。程序会会验证一下i < 13是否满足,13<13是假,所以不执行循环体了,将退出循环。最终输出输出结果为:1、5、9

Next, do a few questions.

Topic 1:

    for (var=1;<10;=+3{        =+1;        console.log(i);    }

Output results: 2, 6, 10

Topic 2:

    for (var=1;<=10; i++{    }    console.log(i);

Output results: 11

Topic 3:

    for(var=1;<7;=+3){    }    console.log(i);

Output results: 7

Topic 4:

    for (var=1;>0; i++{        console.log(i);    }

The dead loop.

Algorithm problem Exercises



Function: is to encapsulate some statements, and then execute them in the form of calls .

Role of the function:

    • Write a large number of repeated statements in the function, and later when you need these statements, you can call the function directly, avoid duplication of work.

    • Simplify programming and make programming modular.

Let's look at an example:

    console.log("你好");    sayHello();     //调用函数    //定义函数:    functionsayHello(){        console.log("欢迎");        console.log("welcome");    }
First step: definition of function

Syntax for function definitions:

    function 函数名字(){    }

The explanations are as follows:

    • Function: is a keyword. Chinese is "function", "function".

    • Function name: The naming convention is the same as the naming of variables. Can only be letters, numbers, underscores, dollar signs, and cannot start with a number.

    • Parameters: There is a pair of parentheses behind, which is used for parameters.

    • Inside the curly braces is the statement of this function.

Step Two: Call the function

Syntax for function calls:

Parameters of the function: formal parameters and arguments

The parameters of the function include the formal parameter and the actual parameter. Take a look at the following diagram to understand:

Note: The number of actual and formal parameters should be the same.


        sum(3,4);        sum("3",4);        sum("Hello","World");        //函数:求和        functionsum(a,{            console.log+ b);        }

Console output Results:

    7    34    helloworld
return value of the function


        console.log(sum(3,4));        //函数:求和        functionsum(a,{            return+ b;        }

The function of return is to end the method.

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JavaScript syntax explained: If statement &for loops & Functions

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