Js: a simple method for determining the length of a calculated string/null _ javascript skills

This article introduces a simple method for determining the length of a string to be null. For more information about how to calculate the length of a string, see the following three methods:

The Code is as follows:

Echo "$ str" awk '{print length ($0 )}'
Expr length "$ str"
Echo "$ str" wc-c

However, if the value obtained in the third method is 1 more, the terminator may be included.
There are three methods to judge whether a string is null:

The Code is as follows:

If ["$ str" = ""]
If [x "$ str" = x]
If [-z "$ str"]

Note: Double quotation marks must be replaced. Otherwise, some commands may report errors!