JavaScript中forEach方法如何与三元运算符及if语句配合使用？

Hey there! Let's break down exactly why those frustrating SyntaxErrors are popping up and get your code running smoothly.

What's Wrong with Line 3? #

Your third line of code looks like this:

let checkArr = (arr) => arr.forEach(el => if (typeof el !== 'string') { console.log(`Type error: ${el} should be a string!`)})

The core issue here is how arrow functions work with concise syntax (no curly braces {}). When you omit curly braces for an arrow function, its body can only contain an expression—not a full statement like if. The if keyword kicks off a statement, which violates this rule and triggers the Unexpected token if error.

To fix this, wrap the inner arrow function's body in curly braces to use a block statement:

let checkArr = (arr) => arr.forEach(el => {
  if (typeof el !== 'string') { 
    console.log(`Type error: ${el} should be a string!`)
  }
})

What's Wrong with Line 4? #

Your fourth line has two small but critical syntax issues:

let checkArr = ((arr) => { arr.forEach(el => typeof el !== 'string' ? console.log(`Type error: ${el} should be a string!`) : null; )})

1. Extra semicolon: Inside the forEach arrow function, you added a semicolon after null. Since this is a concise arrow function using an expression (the ternary operator), that semicolon doesn't belong here—it's what causes the missing ) after argument list error.
2. Redundant outer parentheses: The wrapping ((arr) => { ... }) is unnecessary; you can just use (arr) => { ... } directly.

Here's the cleaned-up version:

let checkArr = (arr) => { 
  arr.forEach(el => 
    typeof el !== 'string' ? console.log(`Type error: ${el} should be a string!`) : null
  )
}

And since the null case does nothing, you can simplify it even further by ditching the ternary entirely (this is more readable anyway):

let checkArr = (arr) => { 
  arr.forEach(el => {
    if (typeof el !== 'string') {
      console.log(`Type error: ${el} should be a string!`)
    }
  })
}

Quick Best Practice Note #

You're redeclaring checkArr three times with let—while this isn't a syntax error, it's messy. You only need to declare checkArr once, then assign your corrected function to it.

Full Working Code #

Putting it all together, here's your code without any syntax errors:

let arr = ['Custy Stomer', 'C. Oostomar', 'C.U.S. Tomer', 3432434, 'Custo Mer', 'Custopher Ustomer', 3432435, 'Kasti Yastimeur'];
let checkArr = (arr) => { 
  arr.forEach(el => {
    if (typeof el !== 'string') {
      console.log(`Type error: ${el} should be a string!`)
    }
  })
}
checkArr(arr);

This will correctly log the type errors for the numeric elements in your array.

内容的提问来源于stack exchange，提问作者v01d