Parameter X implicitly has an 'any' type

This error occurs when you fail to add a type annotation to a parameter in a function.

const addTwoNumbers = (a, b) => {
Parameter 'b' implicitly has an 'any' type.7006
Parameter 'b' implicitly has an 'any' type.
Parameter 'a' implicitly has an 'any' type.7006
Parameter 'a' implicitly has an 'any' type.  return a + b;
};

#

Why Is It Happening?

This is happening because of a setting that most TypeScript projects have turned on - strict mode.

// tsconfig.json
{
  "compilerOptions": {
    "strict": true
  }
}

Strict mode tells TypeScript to add more checks to your code to make sure it's safe. One of these checks is to make sure that all parameters have a type annotation.

Pretty much all modern TypeScript projects use strict mode, so you'll see this error a lot.

#

Why Is This Error Part Of Strict Mode?

TypeScript can't infer the type of a parameter if you don't give it a type annotation. In the example above, TypeScript can't infer from usage what the type of a and b should be.

Adding two parameters together could be a number, sure. But it could be that you want to concatenate two strings together, which is also possible with the + operator.

const concatTwoStrings = (a, b) => {
Parameter 'b' implicitly has an 'any' type.7006
Parameter 'b' implicitly has an 'any' type.
Parameter 'a' implicitly has an 'any' type.7006
Parameter 'a' implicitly has an 'any' type.  return a + b;
};

So, in strict mode TypeScript demands that you add a type annotation to all parameters. This is to make sure your function is called correctly:

const addTwoNumbers = (a: number, b: number) => {
  return a + b;
};

addTwoNumbers(1, 2); // OK
addTwoNumbers("1", "2"); // Error!
Argument of type 'string' is not assignable to parameter of type 'number'.2345
Argument of type 'string' is not assignable to parameter of type 'number'.

#

What Is An Implicit Any?

If you don't have strict mode turned on, TypeScript will automatically assign the any type to parameters that don't have a type annotation.

const addTwoNumbers = (a, b) => {
  return a + b;
};

const result = addTwoNumbers(1, 2);
        
const result: any

This is bad, because any disables type checking on anything it's applied to. We can begin really messing up if we're not careful:

const addTwoNumbers = (a, b) => {
  return a + b;
};

const result = addTwoNumbers(1, 2);

result.touppercase(); // No error!

This code will fail at runtime, but because we're using any it won't yell at us in our IDE.

So, an 'implicit any' is when TypeScript implicitly assigns the any type to a variable or parameter. It's bad, and the noImplicitAny setting is bundled as part of strict mode for that reason.