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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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