JavaScript detects if there are any problems with variables
- 2020-03-27 00:00:10
- OfStack
When writing JavaScript programs, you often need to detect the existence of a variable, which is a very simple task, but if you are not careful, you will encounter some problems. There are mainly some points:
1. Ordinary variables
The 'variable is not defined' error occurs, and the expected dialog pops up if you change it to:
2. Properties of objects
Typeof is not required for properties of the detected object.
3. You run into a similar problem when you add properties to an object
1. Ordinary variables
<script type="text/javascript">
if(variable){
alert('rain man');
}
</script>
The 'variable is not defined' error occurs, and the expected dialog pops up if you change it to:
<script type="text/javascript">
if( typeof variable == 'undefined' ){
alert('rain man');
}
</script>
2. Properties of objects
<script type="text/javascript">
var two = {};
if(two.b){
alert('rain man');
}
if( window.addEventListener ){
alert('This is not IE!');
}
</script>
Typeof is not required for properties of the detected object.
3. You run into a similar problem when you add properties to an object
<script type="text/javascript">
var obj = {};
obj.property.number = 2; //The 'obj.property is undefined' error will appear
var obj = {};
obj.property = 2 ;
obj.property.number = 3;
</script>