iD/js/id/modes/add_line.js

iD.modes.AddLine = function(context) {
    var mode = {
        id: 'add-line',
        button: 'line',
        title: t('modes.add_line.title'),
        description: t('modes.add_line.description'),
        key: '3'
    };

    var behavior = iD.behavior.AddWay(context)
            .on('start', start)
            .on('startFromWay', startFromWay)
            .on('startFromNode', startFromNode);

    function start(loc) {
        var graph = context.graph(),
            node = iD.Node({loc: loc}),
            way = iD.Way();

        context.perform(
            iD.actions.AddEntity(node),
            iD.actions.AddEntity(way),
            iD.actions.AddVertex(way.id, node.id));

        context.enter(iD.modes.DrawLine(context, way.id, 'forward', graph));
    }

    function startFromWay(other, loc, index) {
        var graph = context.graph(),
            node = iD.Node({loc: loc}),
            way = iD.Way();

        context.perform(
            iD.actions.AddEntity(node),
            iD.actions.AddEntity(way),
            iD.actions.AddVertex(way.id, node.id),
            iD.actions.AddVertex(other.id, node.id, index));

        context.enter(iD.modes.DrawLine(context, way.id, 'forward', graph));
    }

    function startFromNode(node) {
        var graph = context.graph(),
            parent = graph.parentWays(node)[0],
            isLine = parent && parent.geometry(graph) === 'line';

        if (isLine && parent.first() === node.id) {
            context.enter(iD.modes.DrawLine(context, parent.id, 'backward', graph));

        } else if (isLine && parent.last() === node.id) {
            context.enter(iD.modes.DrawLine(context, parent.id, 'forward', graph));

        } else {
            var way = iD.Way();

            context.perform(
                iD.actions.AddEntity(way),
                iD.actions.AddVertex(way.id, node.id));

            context.enter(iD.modes.DrawLine(context, way.id, 'forward', graph));
        }
    }

    mode.enter = function() {
        context.install(behavior);
        context.tail(t('modes.add_line.tail'));
    };

    mode.exit = function() {
        context.uninstall(behavior);
    };

    return mode;
};