Branching
Branching statements determine how a program makes decisions and repeats actions.
By evaluating conditions (true or false statements), a program can choose different paths of execution. Those paths of execution are represented by blocks, which are groups of instruction enclosed in curly braces ({ and }).
If you are familiar with JavaScript or a similar programming language like C++ or Java, you are most likely familiar with all of the branching statements listed here.
If statements
An if statement executes a block of instructions if the given condition is true. A block is a group of instructions in curly braces ({ and }), and a condition is any true or false value.
Consider the simple program below, which only performs the console.log instruction if the value in score is greater than 80.
Recall from the basics that JavaScript considers empty strings and zero to be false values, and non-empty strings/non-zero values to be true values.
Else-if statements
An else if statement after an if statement provides an alternate condition and set of instructions to execute.
The condition is only evaluated if the condition in the previous if statement evaluated to false. There can be
any number of else if statements after an if statement.
Typescript can make further inference on the data type of a value based on the evaluation of if and else if statements.
Else statements
An else statement indicates an alternate group of instructions to perform if no preceding if or else if condition was true.
An else statement must always have a corresponding if statement.
Notice how the program below works correctly for all possible values except for 7. The else statement is only paired with the single if statement that precedes it.
While loops
A while statement repeats a block of instructions as long as a given condition is true.
A while loop generally has three components - the initialization, the condition, and the increment.
It is important that the condition eventually becomes false, or the program will endlessly repeat the instructions and not do any further work.
For loops
A for statement is a concise way to write the initialization, condition, and increment of a loop statement.
A common use of a for loop is to concisely express the iteration of a while loop, as shown below.
Do while loop
A do...while statement executes a block of instructions at least once before checking the condition to determine whether the block should be repeated. It is often used in cases where the loop body must execute at least once, regardless of the condition.
Switch statement
The switch statement selects instructions from a list of case statements.
TypeScript can help you write switch statements and ensure that all cases are handled.
Notice that if you change the value to a string, it performs both console.log statements. Until a switch statement encounters a break statement, it will continue to perform the instructions of each subsequent case.
Try and catch
When an instruction might produce an error, a try and catch statement can define some custom error handling logic.
When a try-catch statement is executed, the instructions in the try block are executed in order. If any instruction produces an error, the catch block is executed with error as the first parameter.