While the default target is ES3, the great majority of current browsers support ES2015. Most developers
can therefore safely specify ES2015 or above as a target, unless compatibility with certain ancient
browsers is important.
Strictness
Different users come to TypeScript looking for different things in a type-checker. Some people are
looking for a more loose opt-in experience which can help validate only some parts of their
program, and still have decent tooling. This is the default experience with TypeScript, where types
are optional, inference takes the most lenient types, and there's no checking for potentially
null / undefined values. Much like how tsc emits in the face of errors, these defaults are put in
place to stay out of your way. If you're migrating existing JavaScript, that might be a desirable first
step.
In contrast, a lot of users prefer to have TypeScript validate as much as it can straight away, and
that's why the language provides strictness settings as well. These strictness settings turn static
type-checking from a switch (either your code is checked or not) into something closer to a dial. The
further you turn this dial up, the more TypeScript will check for you. This can require a little extra
work, but generally speaking it pays for itself in the long run, and enables more thorough checks
and more accurate tooling. When possible, a new codebase should always turn these strictness
checks on.
TypeScript has several type-checking strictness flags that can be turned on or off, and all of our
examples will be written with all of them enabled unless otherwise stated. The strict flag in the
CLI, or "strict": true in a tsconfig.json toggles them all on simultaneously, but we can
opt out of them individually. The two biggest ones you should know about are noImplicitAny
and strictNullChecks.
noImplicitAny
Recall that in some places, TypeScript doesn't try to infer types for us and instead falls back to the
most lenient type: any . This isn't the worst thing that can happen - after all, falling back to any is
just the plain JavaScript experience anyway.
function greet(person, date) {
console.log(`Hello ${person}, today is ${date.toDateString()}!`);
}
greet("Maddison", new Date());
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