_javascript techniques for determining whether a function or variable exists in JavaScript

First, whether there is a specified function

Copy Code code as follows:

function Isexitsfunction (funcName) {
try {
if (typeof eval (funcName) = = "function") {
return true;
}
catch (e) {}
return false;
}

Second, similar to PHP commonly used to determine whether the existence of a function, does not exist to create

Copy Code code as follows:

if (typeof String.prototype.endsWith!= ' function ') {
String.prototype.endsWith = function (suffix) {
return this.indexof (suffix, this.length-suffix.length)!==-1;
};
}

Third, to determine whether the JS function exists, if the existence of the execution

Assuming that funcname is the function name, you can achieve the goal by using the following method

Be sure to add a try catch block, otherwise it will not work.

Copy Code code as follows:

Try
{
if (typeof eval (funcName) = = "function")
{
FuncName ();
}
}catch (e)
{
Alert ("not function");
}

Four, whether the existence of the specified variable

Copy Code code as follows:

function Isexitsvariable (variableName) {
try {
if (typeof (variableName) = = "undefined") {
Alert ("value is undefined");
return false;
} else {
Alert ("value is true");
return true;
}
catch (e) {}
return false;
}