How to determine whether functions and variables exist in JavaScript

Methods to determine whether functions and variables exist in JavaScript: 1. To determine whether the specified function exists, the code is [if (typeof(eval(funcName)) == "function")]; 2. To determine whether it exists Specify variables.

The operating environment of this tutorial: Windows 7 system, JavaScript version 1.8.5, DELL G3 computer.

Methods to determine whether functions and variables exist in JavaScript:

1. Whether the specified function exists

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

2. Similar to the commonly used judgment functions in PHP Whether it exists, create it if it does not exist

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

3. Determine whether the js function exists, if it exists, execute it

Assuming that funcName is the function name, use the following method to achieve the goal

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

try
{ 
 if(typeof(eval(funcName))=="function") 
 {
   funcName();
 }
}catch(e)
{
//alert("not function"); 
}

4. Whether the specified variable exists

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;
}

Under normal circumstances, we use

if("undefined" != typeof downlm){ 
if(downlm=="soft"){ 
document.write('成功'); 
} 
}

to determine whether a variable exists alone. Related free learning recommendations: javascript(Video)

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