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#JavaScript ES6, 90 Bytes

f=a=>{i=new Image();i.src="//placehold.it/1x1";i.onload=b=>a(true);i.onerror=c=>a(false);} 

#JavaScript ES6, 22 21 bytes (Invalid)

Some browsers don't fully support, or produce the expected result when using navigator.onLine.

f=a=>navigator.onLine 

#JavaScript ES6, 90 81 Bytes

f=a=>{i=new Image();i.src="//placehold.it/1x1";i.onload=b=>a(1);i.onerror=c=>a()} 

#JavaScript ES6, 22 21 bytes (Invalid)

Some browsers don't fully support, or produce the expected result when using navigator.onLine.

f=a=>navigator.onLine 

#JavaScript ES6, 22 21 bytes

f=a=>navigator.onLine 

#JavaScript ES6, 90 Bytes

f=a=>{i=new Image();i.src="//placehold.it/1x1";i.onload=b=>a(true);i.onerror=c=>a(false);} 

#JavaScript ES6, 22 21 bytes (Invalid)

Some browsers don't fully support, or produce the expected result when using navigator.onLine.

f=a=>navigator.onLine 

#JavaScript ES6, 22 bytes

f=()=>navigator.onLine 

#JavaScript ES6, 22 21 bytes

f=a=>navigator.onLine