TypeError: .toLowerCase is not a function occurs when we call toLowerCase() function on object which is not an string. toLowerCase() function can be only called on string. To resolve this issue, convert value to string using toString() ,method before calling toLowerCase() method.
Let’s see with help of simple example
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const s = {}; // will throw TypeError: str.toLowerCase is not a function const lowerCaseStr = s.toLowerCase(); |
You will get following error when you execute the program:
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const lowerCaseStr = s.toLowerCase(); ^ TypeError: s.toLowerCase is not a function at Object.<anonymous> (HelloWorld.js:4:24) at Module._compile (internal/modules/cjs/loader.js:959:30) at Object.Module._extensions..js (internal/modules/cjs/loader.js:995:10) at Module.load (internal/modules/cjs/loader.js:815:32) at Function.Module._load (internal/modules/cjs/loader.js:727:14) at Function.Module.runMain (internal/modules/cjs/loader.js:1047:10) at internal/main/run_main_module.js:17:11 |
We got this error because str is not an string here.
Convert object to string using toString() method and it will work fine.
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const n= 123 const lowerCaseStr = n.toString().toLowerCase(); |
Output:
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123 |
You can also check if it is type of string before calling toLowerCase() to make sure that toLowerCase() is being called on string only.
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var num=123; num=typeof num === 'string' ?num.toLowerCase():''; console.log(num); |
Output:
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As you can see num is not a string, so result is empty string.
We used ternary operator to check if num is string or not. If it is a string, call the toLowerCase() method, otherwise return empty string.
Read also: map is not a function in JavaScript.
That’s all about how to resolve TypeError: toLowerCase is not a function in JavaScript.
