Dr. Axel Rauschmayer - Deep JavaScript

For example, you may see:

That is called the type signature of Number.isFinite(). This notation, especially the static types number of num and boolean of the result, are not real JavaScript. The notation is borrowed from the compile-to-JavaScript language TypeScript (which is mostly just JavaScript plus static typing).

Why is this notation being used? It helps give you a quick idea of how a function works. The notation is explained in detail in a 2ality blog post, but is usually relatively intuitive.

• Thanks to Allen Wirfs-Brock for his advice via Twitter and blog post comments. It helped make this book better.

• More people who contributed are acknowledged in the chapters.

• 2.1
  • 2.1.1
• 2.2
  • 2.2.1
  • 2.2.2
  • 2.2.3
  • 2.2.4
  • 2.2.5
• 2.3
• 2.4
  • 2.4.1
  • 2.4.2
  • 2.4.3
  • 2.4.4
• 2.5
  • 2.5.1
  • 2.5.2
• 2.6

Each operation (function, operator, etc.) expects its parameters to have certain types. If a value doesnt have the right type for a parameter, three common options for, e.g., a function are:

1. The function can throw an exception:

   }

2. The function can return an error value:

   }

3. The function can convert its arguments to useful values:

   }

In (3), the operation performs an implicit type conversion. That is called type coercion.

JavaScript initially didnt have exceptions, which is why it uses coercion and error values for most of its operations:

However, there are also cases (especially when it comes to newer features) where it throws exceptions if an argument doesnt have the right type:

• Accessing properties of null or undefined:

  TypeError: Cannot use 'in' operator to search for 'prop' in null

• Using symbols:

  TypeError: Cannot convert a Symbol value to a number

• Mixing bigints and numbers:

  TypeError: Cannot mix BigInt and other types

• New-calling or function-calling values that dont support that operation:

  TypeError: (intermediate value) is not a constructor

• Changing read-only properties (only throws in strict mode):

  TypeError: Cannot assign to read only property 'prop'