This will log both Number is less than 10! and then Logged after if statement. Since num < 10 is true, the code inside the 'if' statement runs, and then the code afterwards always runs.
Now try changing num to initialise to 15. The condition num < 10 is now false, so the code inside the 'if' statement does not run. Therefore the only message logged is Logged after if statement, since that comes after the 'if' statement.
Similar to the ?: conditional operator, the 'if' statement condition can be any value and it will be interpreted as either truthy or falsy. For example you could use just if (num), and this will count as false if num is zero or NaN, otherwise true.
Else
'If' statements can also be immediately followed by an 'else' statement. The code inside the 'else' statement runs if the condition is false (or falsy). Try the following code snippet.
let num = 15;
if (num < 10)
{
console.log("Number is less than 10!");
}
else
{
console.log("Number is greater or equal to 10!");
}
Since num is initialised to 15, the condition num < 10 is false, and so the code in the 'else' statement runs, logging: Number is greater or equal to 10!
When there is both an 'if' and an 'else' statement, one of the two sections is guaranteed to run, as they correspond to the two possible outcomes of the condition.
Else if
Multiple conditions can be chained using the else if pattern like so.
let num = 15;
if (num < 10)
{
console.log("Number is less than 10!");
}
else if (num < 20)
{
console.log("Number is between 10 and 20!");
}
else
{
console.log("Number is greater or equal to 20!");
}
Try changing the initial value of num to get each message to appear.
Optional braces
You can put as many statements as you like in between the { and } braces, including further 'if' statements. However if there is only a single statement in between the braces, the braces can be omitted. For example this:
if (num < 10)
{
console.log("Number is less than 10!");
}
if (num < 10)
console.log("Number is less than 10!");
as there is only a single statement. This also applies to 'else' and 'else if' chains, so for example this:
let num = 15;
if (num < 10)
{
console.log("Number is less than 10!");
}
else if (num < 20)
{
console.log("Number is between 10 and 20!");
}
else
{
console.log("Number is greater or equal to 20!");
}
let num = 15;
if (num < 10)
console.log("Number is less than 10!");
else if (num < 20)
console.log("Number is between 10 and 20!");
else
console.log("Number is greater or equal to 20!");
Whitespace
In general, TypeScript doesn't care about whitespace (i.e. spaces and tabs). This allows a lot of flexibility in how code is laid out. The following code snippet shows several ways to write an 'if' statement, all of which are equivalent.
if (num < 10)
{
console.log("Number is less than 10!");
}
if (num < 10) {
console.log("Number is less than 10!");
}
if (num < 10)
console.log("Number is less than 10!");
if(num<10)console.log("Number is less than 10!");
if (num < 10) { console.log("Number is less than 10!"); }
if (num < 10)
{
console.log("Number is less than 10!");
}
In practice it's best to stick to a sensible style (avoiding the extremes of adding lots of spaces or removing as many spaces as possible), and just be consistent about using the same style. This guide will stick to the first form above.
Prefer braces
While omitting the braces is sometimes a useful shorthand, it's generally preferable to include them. This helps keep the code clear and readable and avoid mistakes.
One common mistake omitting braces comes from the fact TypeScript ignores indentation - it's only there for readability. Therefore if you write code like this:
if (num < 10)
console.log("First message!");
console.log("Second message!");
This might look like both messages are logged if number < 10, but it is actually equivalent to this:
if (num < 10)
{
console.log("First message!");
}
console.log("Second message!");
This is because when the braces are omitted, only the single following statement counts as inside, regardless of indent. Preferring to use braces helps avoid this potential mistake.
Prefer 'if' statements to ?:
It's sometimes possible to use either an 'if' statement or the conditional ?: operator. For example the following 'if' statement:
let num = 5;
let message: string = "";
if (num < 5)
{
message = "Number is less than 5!";
}
else
{
message = "Number is greater or equal to 5!";
}
console.log(message);
let num = 5;
let message = num < 5 ? "Number is less than 5!" : "Number is greater or equal to 5!";
console.log(message);
Despite the latter being fewer lines of code, the former is generally regarded as more readable. So if in doubt prefer to use 'if' statements. The conditional operator has its uses, but is best when all three parts are very short and simple.