[Effective JavaScript notes] 9th: always declare local variables

If you forget to declare a variable as a local variable, the variable will be implicitly converted to a global variable

Function Swap (A,I,J) {

Temp=a[i];

A[I]=A[J];

A[j]=temp;

}

Although the program does not declare the TEMP variable using VAR, execution does not go wrong, and temp is added to the global usage domain and becomes a global variable.

Code that has been revised

Function Swap (A,I,J) {

var temp=a[i];

A[I]=A[J];

A[j]=temp;

}

Intentional creation of global variables is a bad style, and accidental creation is a disaster.

You can use the Lint tool to detect code. Using the tool can improve the quality of your JS code, it is worth spending time to spend a bit. Many small error problems can be avoided.

Tips

• Always declare a new local variable with VAR
• Consider using tools such as lint to help examine the variables of your code

[Effective JavaScript notes] 9th: always declare local variables