Differences between substr and substring in Javascript

Today, when I wrote a JS file for uploading images to implement local browsing, I used substr to write the results and wrote them as substring, at last, I found a difference with what I imagined. First glance CodeI found that I wrote substr as substring, so it didn't achieve the expected results! Check the differences between the two:

String. substr (N1, N2) is a string that is commonly used to extract the specified length (N2) from the specified position (N1;
String. substring (N1, N2), which is commonly used from the specified position (N1) Specified location(N2) string;

For example:
Alert ("123456789". substr (34567) it displays"
Alert ("123456789". substring (345) is displayed"