Posted By
Bajarangi soft ,
Posted On 10-10-2020
Accidentally Using the Assignment Operator
JavaScript programs may generate unexpected results if a programmer accidentally uses an assignment operator (=), instead of a comparison operator (==) in an if statement.
This if statement returns false (as expected) because x is not equal to 10:
Example(1)
<script>
var x = 0;
document.getElementById("demo").innerHTML = Boolean(x == 10);
</script>
This if statement returns true (maybe not as expected), because 10 is true:
Example(2)
<script>
var x = 0;
document.getElementById("demo2").innerHTML = Boolean(x = 10);
</script>
This if statement returns false (maybe not as expected), because 0 is false:
<script>
var x = 0;
document.getElementById("demo3").innerHTML = Boolean(x = 0);
</script>
An assignment always returns the value of the assignment.
Expecting Loose Comparison
In regular comparison, data type does not matter. This if statement returns true:
Example(4)
<script>
var x1 = 10;
var y1 = "10";
document.getElementById("demo4").innerHTML = Boolean(x1 == y1);
</script>
if statement returns false:
<script>
var x1 = 10;
var y1 = "10";
document.getElementById("demo5").innerHTML = Boolean(x1 === y1);
</script>
It is a common mistake to forget that switch statements use strict comparison:
This case switch will display an alert:
Example(5)
<script>
var x = 10;
switch(x) {
case 10: alert("Hello");
}
</script>
This case switch will not display an alert:
Example(6)
<script>
var x = 10;
switch(x) {
case "10": alert("Hello");
}
</script>
Confusing Addition & Concatenation
Addition is about adding numbers.
Concatenation is about adding strings.
In JavaScript both operations use the same + operator.
Because of this, adding a number as a number will produce a different result from adding a number as a string:
Example(7)
<script>
var x = 10 + "5";
document.getElementById("demo").innerHTML = x;
</script>
<script>
var x = 10;
var y = "5";
var z = x + y;
document.getElementById("demo").innerHTML = z;
</script>
Misunderstanding Floats
All numbers in JavaScript are stored as 64-bits Floating point numbers (Floats).
All programming languages, including JavaScript, have difficulties with precise floating point values:
Example(9)
<script>
var x = 0.1;
var y = 0.2;
var z = x + y;
document.getElementById("demo").innerHTML = z;
</script>
<script>
var x = 0.1;
var y = 0.2;
var z = (x * 10 + y *10) / 10;
document.getElementById("demo").innerHTML = z;
</script>
<!DOCTYPE html>
<html>
<head>
<title>What Are The Common Mistakes Done With Java Script</title>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1">
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.4.1/css/bootstrap.min.css">
</head>
<style>
body{
background: black;
}
</style>
<body>
<div class="container">
<br>
<div class="text-center">
<h1 id="color" style="color: White">What Are The Common Mistakes Done With Java Script</h1>
</div>
<br>
<br>
<div class="well">
<h2 id="demo1"></h2>
<h2 id="demo2"></h2>
<h2 id="demo3"></h2>
<h2 id="demo4"></h2>
<h2 id="demo5"></h2>
<script>
var x = 0;
document.getElementById("demo1").innerHTML = Boolean(x == 10);
document.getElementById("demo2").innerHTML = Boolean(x = 10);
document.getElementById("demo3").innerHTML = Boolean(x = 0);
var x1 = 10;
var y1 = "10";
document.getElementById("demo4").innerHTML = Boolean(x1 == y1);
document.getElementById("demo5").innerHTML = Boolean(x1 === y1);
</script>
</div>
</div>
</body>
</html>