function - Why does typescript allow default parameters before required parameters? -

i noticed , function (using default params) doesn't cause error on compilation.

function buildaddress(address1 = 'n/a', address2: string) {         displayaddress( address1 +' '+ address2);     } 

but function (using optional params) does.

function buildaddress(address1?: string, address2: string) {         displayaddress( address1 +' '+ address2);     } 

why ?

i'm surprised of behavior, normal ? have benefit ? feature or bug ?

have tried use first version without passing first argument?

function buildaddress(address1: string = 'n/a', address2: string) {     console.log(address1, address2); }  buildaddress("address2"); 

results in:

supplied parameters not match signature of call target

if put default value 2nd parameter though:

function buildaddress(address1: string , address2: string = 'n/a') {     console.log(address1, address2); } 

it works.

adding default value first parameter helps in case pass undefined:

buildaddress(undefined, "address2"); 

as compiles to:

function buildaddress(address1, address2) {     if (address1 === void 0) { address1 = 'n/a'; }     console.log(address1, address2); } 

so in reality if you're doing first parameter isn't optional @ all, must pass value, , default value if pass undefined.
compiler won't complain function signature because first parameter has value sure, in 2nd function, since first param optional compiler complains.

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edit

this behvior can used safe guard against undefined values, example:

function buildaddress(address1 = 'n/a', address2: string) {     displayaddress(address1 + ' ' + address2); }  function getaddress1(): string {     // logic here, might return undefined }  buildaddress(getaddress1(), "address 2"); 

i'm not sure if design or byproduct, it's useful in cases.