Value.

  "value" can be used with "time" like this.
Witch mean you can keyframe on top of the motion the rest of the expression do.

FX like this: 

 time*200+value

5 Amazing Expressions in After Effects

Offset - Index - Linear - Clamp - Value At Time

0:47 Offset/Move The Keyframes 3:45 Index/I Got Your Number
6:57 Linear/Music Time 11:55 Clamp/Let’s Not Get Crazy Here 15:07 Value At Time/ Lagging Motion

https://youtu.be/vQzVmae72wM

Lol, well thats the web skilles of 2020

Dimensions of a text or layer

The expression allows you to calculate the dimensions of the text or any other layer.
Can be useful for creating a substrate that will automatically change size,
depending on the width of the text.

title = thisComp.layer("Text Layer") ; // Layer
indW = 10; // Spacing width
indH = 10; // Spacing  height
rect = title.sourceRectAtTime(time,false);
x = rect.width+indW;
y = rect.height+indH;
[x,y]

Type on text with blinking cursor

The expression creates a typing animation with a blinking cursor. 

Applies to the Source Text parameter of the text layer. 

Speed = 10; // Speed typing
Blink = 2; // The speed of blinking cursor
CursorType = "|" // Type of the cursor. This is usually a vertical line "|" or the bottom dash "_"

F = Math.round(time*Blink % 1);
L = text.sourceText.length;
T = (time - thisLayer.inPoint)*Speed;
if(F ==1 | (T0) ){Fl=CursorType;}else{Fl="";}
substr(0,T) + Fl

Loop wiggle 2

With this expression by Dan Ebberts, you can loop a shake animation for a given duration.

freq = 1; // Frequency
amp = 110; // Amplitude
loopTime = 3; // Cycle time (in seconds)
t = time % loopTime;
wiggle1 = wiggle(freq, amp, 1, 0.5, t);
wiggle2 = wiggle(freq, amp, 1, 0.5, t - loopTime);
linear(t, 0, loopTime, wiggle1, wiggle2)

Rebound bounce

With this expression, you can create a rebound effect of the object from the floor. Allows you to animate almost any parameter.

e = .3; // Bouncing
g = 1; // Gravity
nMax = 10; // Maximum bounces

n = 0;
if (numKeys > 0){
n = nearestKey(time).index;
if (key(n).time > time) n--;
}
if (n > 0){
t = time - key(n).time;
v = -velocityAtTime(key(n).time - .001)*e;
vl = length(v);
if (value instanceof Array){
vu = (vl > 0) ? normalize(v) : [0,0,0];
}else{
vu = (v < 0) ? -1 : 1;
}
tCur = 0;
segDur = 2*vl/(g*1000);
tNext = segDur;
nb = 1; // number of bounces
while (tNext < t && nb <= nMax){
vl *= e;
segDur *= e;
tCur = tNext;
tNext += segDur;
nb++
}
if(nb <= nMax){
delta = t - tCur;
value +  vu*delta*(vl - (g*1000)*delta/2);
}else{
value
}
}else
value

Damping - Bounce

One of the most popular expressions for After Effects. It allows animate virtually any parameter with a damping effect.

amp = .04;// The higher the value, the greater the amplitude
freq = 2;// The higher the value, the higher the frequency
decay = 5;// The higher the value, the smaller the delay

n = 0;
if (numKeys > 0){
n = nearestKey(time).index;
if (key(n).time > time){
n--;
}
}
if (n == 0){
t = 0;
}else{
t = time - key(n).time;
}
if (n > 0){
v = velocityAtTime(key(n).time - thisComp.frameDuration/10);
value + v*amp*Math.sin(freq*t*2*Math.PI)/Math.exp(decay*t);
}else{
value;
}