TypeError: startsWith 不是 JavaScript 中的函数

TypeError: startsWith 不是 JavaScript 中的函数

TypeError: startsWith is not a function in JavaScript

“TypeError: startsWith is not a function”错误发生在我们
startsWith()对一个不是字符串的值调用方法时。

要解决该错误，请使用该toString()
方法将值转换为字符串，或确保仅对startsWith字符串调用该方法。

下面是错误如何发生的示例。

const str = 123;

// ⛔️ TypeError: startsWith is not a function
const result = str.startsWith('1');

我们对一个数字调用了
String.startsWith
方法并返回了错误。

startsWith只在字符串上调用方法

要解决该错误，请确保仅对startsWith()字符串调用该方法。您可以使用toString()方法将大多数值转换为字符串。

const str = 123;

const result = str.toString().startsWith('1');

console.log(result); // 👉️ true

您还可以使用String()构造函数将值转换为字符串。

const str = 123;

const result = String(str).startsWith('1');

console.log(result); // 👉️ true

构造String()函数将提供的值转换为字符串并返回结果。

string在调用之前检查值是否是类型startsWith()

startsWith()或者，您可以在调用该方法之前检查该值是否为字符串
。

const str = null;

const result = typeof str === 'string' ? str.startsWith('a') : false;

console.log(result); // 👉️ false

我们使用三元运算符来检查str变量是否存储字符串。

如果是，则返回逗号左边的值，否则返回右边的值。

您也可以使用简单的if语句来获得相同的结果。

const str = null;

let result = false;

if (typeof str === 'string') {
  result = str.startsWith('a');
}

console.log(result); // 👉️ false

If the error persists, console.log the value you’re calling the startsWith
method on and check its type using the typeof operator, e.g.
console.log(typeof myStr).

console.log(typeof 'bobbyhadz.com'); // 👉️ string
console.log(typeof {}); // 👉️ object
console.log(typeof 123); // 👉️ number

If the value is an object, there’s a very good chance that you are forgetting to
access a specific property on which you need to call the startsWith() method.

// ✅ with objects
const obj = {
  name: 'bobby',
};

const result1 = obj.name.startsWith('bo');
console.log(result1); // 👉️ true

// -----------------------------------------
// ✅ with arrays

const arr = ['bobby', 'hadz', 'com'];

const result2 = arr[0].startsWith('bo');
console.log(result2); // 👉️ true

We accessed a property on the object and an element in the array before calling
the startsWith() method.

Check if all strings in an array start with a specific substring #

If you want to check if all strings in an array start with a specific substring,
use the every() method to iterate over the array and then call the
startsWith method on each string.

const arr = ['ab', 'ac', 'ad'];

const result = arr.every(str => str.startsWith('a'));

console.log(result); // 👉️ true

On each iteration, we call the startsWith method on the string and return the
result.

Conclusion #

The “TypeError: startsWith is not a function” error occurs when we call the
startsWith() method on a value that is not a string.

To solve the error, convert the value to a string using the toString()
method or make sure to only call the startsWith method on strings.