1. What is Narrowing?

• The process of collapsing data types
• Using conditions to bind data types
• Helps to better understand data types => write safer code and avoid manipulation errors

2. Techniques:

The narrowing techniques in Typescript allow working with precise data types and provide automation for the data checking process, helping you to detect and correct errors quickly.

2.1 Type guards

Use typeof , instanceof , in , or Array.isArray() conditions to narrow down the return data type

Error because " ".repeat() accepts number data, while we pass number | string => Right way

2.1.1 typeof Type Guard

• How to check data type based on typeof
• TS reports an error if the returned data type does not match the declared data type

When we check the condition that strs is an object data type or not, we can select string[] as an array. But there is a problem that typeof null is also object . Therefore, Typescript can only narrow it down to the string[] | null data type string[] | null instead of null as desired. However, we also have another way of checking called truthiness checking.

2.1.2 Truth narrowing

• Here is how to shorten type based on truthy or falsy value
• May lead to errors if the case is not handled properly
• Helps to write safer and more accurate code

Here, strs is null error like when using only typeof by checking strs . Alternatively, we can check with !

2.1.3 Equality narrowing

• Use === , !== , == , !=
• Type shortening based on a variable value
• Compare with a specific value
• Used for different types such as boolean, number, string or literal type

When checking the value of x , y . TS knows that their data types must also be equal. Therefore, TS also knows that x and y must be of type string in the first branch

2.1.4 in operator

• Check if a certain property exists in an object

2.1.5 instanceof narrowing

• used to check if an object is created from a particular class

2.1.6 Using type predicates

• Helps us determine if a variable or value belongs to a particular type
• Shorten the data type of the variable or value, and can use the variable or value with a more precise data type.