1、Display the typeof all variables types
<!DOCTYPE html>
<html>
<body>
<h2>The JavaScript typeof Operator</h2>
<p>The typeof operator returns the type of a variable, object, function or expression.</p>
<p id="demo"></p>
<script>
document.getElementById("demo").innerHTML =
typeof "john" + "<br>" +
typeof 3.14 + "<br>" +
typeof NaN + "<br>" +
typeof false + "<br>" +
typeof [1,2,3,4] + "<br>" +
typeof {name:'john', age:34} + "<br>" +
typeof new Date() + "<br>" +
typeof function () {} + "<br>" +
typeof myCar + "<br>" +
typeof null;
</script>
</body>
</html>
string
number
number
boolean
object
object
object
function
undefined
object
2、convert a number to a string.
<!DOCTYPE html>
<html>
<body>
<h2>The JavaScript String() Method</h2>
<p>The String() method can convert a number to a string.</p>
<p id="demo"></p>
<script>
var x = 123;
var y = 453;
document.getElementById("demo").innerHTML =
String(x) + "<br>" +
(y+y) + "<br>" +
(y+y.toString()) + "<br>" +
y+y + "<br>" +
String(123) + "<br>" +
String(100 + 23);
</script>
</body>
</html>
3、find out if a variables is an Array.
<!DOCTYPE html>
<html>
<body>
<h2>JavaScript Arrays</h2>
<p>This "home made" isArray() function returns true when used on an array:</p>
<p id="demo"></p>
<script>
var fruits = ["Banana", "Orange", "Apple", "Mango"];
document.getElementById("demo").innerHTML = isArray(fruits);
function isArray(myArray) {
console.info(myArray.constructor) //function Array() { [native code] }
return myArray.constructor.toString().indexOf("Array") > -1;
}
</script>
</body>
</html>
<script>
var myDate = new Date();
document.getElementById("demo").innerHTML = isDate(myDate);
function isDate(myDate) {
return myDate.constructor.toString().indexOf("Date") > -1;
}
</script>