Use the MAP Extend Tool to extend lines to a selected intersecting line.
The MAP Extend Tool works on all lines (paths and compound paths).
The MAP Extend Tool doesn't work on closed paths. As a workaround, if necessary, expand a closed path first, then extend.
Using the MAP Extend Tool
A line can only be extended if it will intersect the selected "extend-to" line. In the example below, the dotted line represents how the existing line will intersect the extend-to line.
Step 1: Select the extend-to line.
On the Adobe Illustrator Tools toolbar, click the MAP Extend Tool button. Click the line to be extended (the extension will have to intersect the extend-to line).
Step 2: Click a line to extend it.
It is necessary to click the portion of the line closest to the extend-to line. Clicking the portion of the line furthest from the extend-to line will not extend the line.
The line is extended to the selected extend-to line. It is considered as the same path as the original line, not an additional segment.
Result: Line is extended to the selected line.
To quickly select other lines while the MAP Extend Tool is enabled, hold the Ctrl or Command (Mac) key to change to the Selection Tool. To select multiple lines, hold both Ctrl (or Command) and Shift keys.