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First, the code in try {…} is executed.
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If there were no errors, then catch(err) is ignored: the execution reaches the end of try and goes on, skipping catch.
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If an error occurs, then the try execution is stopped, and control flows to the beginning of catch(err). The err variable (we can use any name for it) will contain an error object with details about what happened.
Example:
an errorless program:
try {
alert('Start of try runs'); // (1) <--
// ...no errors here
alert('End of try runs'); // (2) <--
} catch(err) {
alert('Catch is ignored, because there are no errors'); // (3)
}
with error:
try {
alert('Start of try runs'); // (1) <--
lalala; // error, variable is not defined!
alert('End of try (never reached)'); // (2)
} catch(err) {
alert(`Error has occurred!`); // (3) <--
}
real life example:
let json = "{ bad json }";
try {
let user = JSON.parse(json); // <-- when an error occurs...
alert( user.name ); // doesn't work
} catch (e) {
// ...the execution jumps here
alert( "Our apologies, the data has errors, we'll try to request it one more time." );
alert( e.name );
alert( e.message );
}